Merge pull request #176146 from pennae/module-docs-markdown

treewide: markdown option docs

doc/build-aux/pandoc-filters/docbook-writer/rst-roles.lua

···
       content = '<refentrytitle>' .. title .. '</refentrytitle>' .. (volnum ~= nil and ('<manvolnum>' .. volnum .. '</manvolnum>') or '')
     elseif elem.attributes['role'] == 'file' then
       tag = 'filename'
+    elseif elem.attributes['role'] == 'command' then
+      tag = 'command'
+    elseif elem.attributes['role'] == 'option' then
+      tag = 'option'
     end
 
     if tag ~= nil then

doc/contributing/contributing-to-documentation.chapter.md

···
 
 As per [RFC 0072](https://github.com/NixOS/rfcs/pull/72), all new documentation content should be written in [CommonMark](https://commonmark.org/) Markdown dialect.
 
-Additionally, the following syntax extensions are currently used:
+Additional syntax extensions are available, though not all extensions can be used in NixOS option documentation. The following extensions are currently used:
 
 - []{#ssec-contributing-markup-anchors}
   Explicitly defined **anchors** on headings, to allow linking to sections. These should be always used, to ensure the anchors can be linked even when the heading text changes, and to prevent conflicts between [automatically assigned identifiers](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/auto_identifiers.md).
···
   This syntax is taken from [MyST](https://myst-parser.readthedocs.io/en/latest/using/syntax.html#targets-and-cross-referencing).
 
 - []{#ssec-contributing-markup-inline-roles}
-  If you want to link to a man page, you can use `` {manpage}`nix.conf(5)` ``, which will turn into {manpage}`nix.conf(5)`.
+  If you want to link to a man page, you can use `` {manpage}`nix.conf(5)` ``, which will turn into {manpage}`nix.conf(5)`. The references will turn into links when a mapping exists in {file}`doc/build-aux/pandoc-filters/link-unix-man-references.lua`.
 
-  The references will turn into links when a mapping exists in {file}`doc/build-aux/pandoc-filters/link-unix-man-references.lua`.
+  A few markups for other kinds of literals are also available:
+
+  - `` {command}`rm -rfi` `` turns into {command}`rm -rfi`
+  - `` {option}`networking.useDHCP` `` turns into {option}`networking.useDHCP`
+  - `` {file}`/etc/passwd` `` turns into {file}`/etc/passwd`
+
+  These literal kinds are used mostly in NixOS option documentation.
 
   This syntax is taken from [MyST](https://myst-parser.readthedocs.io/en/latest/syntax/syntax.html#roles-an-in-line-extension-point). Though, the feature originates from [reStructuredText](https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#role-manpage) with slightly different syntax.
+
+  ::: {.note}
+  Inline roles are available for option documentation.
+  :::
 
 - []{#ssec-contributing-markup-admonitions}
   **Admonitions**, set off from the text to bring attention to something.
···
     - [`note`](https://tdg.docbook.org/tdg/5.0/note.html)
     - [`tip`](https://tdg.docbook.org/tdg/5.0/tip.html)
     - [`warning`](https://tdg.docbook.org/tdg/5.0/warning.html)
+
+  ::: {.note}
+  Admonitions are available for option documentation.
+  :::
 
 - []{#ssec-contributing-markup-definition-lists}
   [**Definition lists**](https://github.com/jgm/commonmark-hs/blob/master/commonmark-extensions/test/definition_lists.md), for defining a group of terms:

lib/default.nix

···
       getValues getFiles
       optionAttrSetToDocList optionAttrSetToDocList'
       scrubOptionValue literalExpression literalExample literalDocBook
-      showOption showFiles unknownModule mkOption mkPackageOption;
+      showOption showFiles unknownModule mkOption mkPackageOption
+      mdDoc literalMD;
     inherit (self.types) isType setType defaultTypeMerge defaultFunctor
       isOptionType mkOptionType;
     inherit (self.asserts)

lib/options.nix

···
     if ! isString text then throw "literalDocBook expects a string."
     else { _type = "literalDocBook"; inherit text; };
 
+  /* Transition marker for documentation that's already migrated to markdown
+     syntax.
+  */
+  mdDoc = text:
+    if ! isString text then throw "mdDoc expects a string."
+    else { _type = "mdDoc"; inherit text; };
+
+  /* For use in the `defaultText` and `example` option attributes. Causes the
+     given MD text to be inserted verbatim in the documentation, for when
+     a `literalExpression` would be too hard to read.
+  */
+  literalMD = text:
+    if ! isString text then throw "literalMD expects a string."
+    else { _type = "literalMD"; inherit text; };
+
   # Helper functions.
 
   /* Convert an option, described as a list of the option parts in to a

nixos/doc/manual/development/option-declarations.section.md

···
 `description`
 
 :   A textual description of the option, in DocBook format, that will be
-    included in the NixOS manual.
+    included in the NixOS manual. During the migration process from DocBook
+    to CommonMark the description may also be written in CommonMark, but has
+    to be wrapped in `lib.mdDoc` to differentiate it from DocBook. See
+    the nixpkgs manual for [the list of CommonMark extensions](
+    https://nixos.org/nixpkgs/manual/#sec-contributing-markup)
+    supported by NixOS documentation.
+
+    New documentation should preferably be written as CommonMark.
 
 ## Utility functions for common option patterns {#sec-option-declarations-util}
 

nixos/doc/manual/from_md/development/option-declarations.section.xml

···
       <listitem>
         <para>
           A textual description of the option, in DocBook format, that
-          will be included in the NixOS manual.
+          will be included in the NixOS manual. During the migration
+          process from DocBook to CommonMark the description may also be
+          written in CommonMark, but has to be wrapped in
+          <literal>lib.mdDoc</literal> to differentiate it from DocBook.
+          See the nixpkgs manual for
+          <link xlink:href="https://nixos.org/nixpkgs/manual/#sec-contributing-markup">the
+          list of CommonMark extensions</link> supported by NixOS
+          documentation.
+        </para>
+        <para>
+          New documentation should preferably be written as CommonMark.
         </para>
       </listitem>
     </varlistentry>

nixos/lib/make-options-doc/default.nix

···
 
   optionsJSON = pkgs.runCommand "options.json"
     { meta.description = "List of NixOS options in JSON format";
-      buildInputs = [ pkgs.brotli ];
+      buildInputs = [
+        pkgs.brotli
+        (let
+          self = (pkgs.python3Minimal.override {
+            inherit self;
+            includeSiteCustomize = true;
+           });
+         in self.withPackages (p: [ p.mistune_2_0 ]))
+      ];
       options = builtins.toFile "options.json"
         (builtins.unsafeDiscardStringContext (builtins.toJSON optionsNix));
     }
···
 
       ${
         if baseOptionsJSON == null
-          then "cp $options $dst/options.json"
+          then ''
+            # `cp $options $dst/options.json`, but with temporary
+            # markdown processing
+            python ${./mergeJSON.py} $options <(echo '{}') > $dst/options.json
+          ''
           else ''
-            ${pkgs.python3Minimal}/bin/python ${./mergeJSON.py} \
+            python ${./mergeJSON.py} \
               ${lib.optionalString warningsAreErrors "--warnings-are-errors"} \
               ${baseOptionsJSON} $options \
               > $dst/options.json

nixos/lib/make-options-doc/mergeJSON.py

···
         result[opt.name] = opt.value
     return result
 
+# converts in-place!
+def convertMD(options: Dict[str, Any]) -> str:
+    import mistune
+    import re
+    from xml.sax.saxutils import escape, quoteattr
+
+    admonitions = {
+        '.warning': 'warning',
+        '.important': 'important',
+        '.note': 'note'
+    }
+    class Renderer(mistune.renderers.BaseRenderer):
+        def _get_method(self, name):
+            try:
+                return super(Renderer, self)._get_method(name)
+            except AttributeError:
+                def not_supported(children, **kwargs):
+                    raise NotImplementedError("md node not supported yet", name, children, **kwargs)
+                return not_supported
+
+        def text(self, text):
+            return escape(text)
+        def paragraph(self, text):
+            return text + "\n\n"
+        def codespan(self, text):
+            return f"<literal>{text}</literal>"
+        def block_code(self, text, info=None):
+            info = f" language={quoteattr(info)}" if info is not None else ""
+            return f"<programlisting{info}>\n{text}</programlisting>"
+        def link(self, link, text=None, title=None):
+            if link[0:1] == '#':
+                attr = "linkend"
+                link = quoteattr(link[1:])
+            else:
+                # try to faithfully reproduce links that were of the form <link href="..."/>
+                # in docbook format
+                if text == link:
+                    text = ""
+                attr = "xlink:href"
+                link = quoteattr(link)
+            return f"<link {attr}={link}>{text}</link>"
+        def list(self, text, ordered, level, start=None):
+            if ordered:
+                raise NotImplementedError("ordered lists not supported yet")
+            return f"<itemizedlist>\n{text}\n</itemizedlist>"
+        def list_item(self, text, level):
+            return f"<listitem><para>{text}</para></listitem>\n"
+        def block_text(self, text):
+            return text
+        def emphasis(self, text):
+            return f"<emphasis>{text}</emphasis>"
+        def strong(self, text):
+            return f"<emphasis role=\"strong\">{text}</emphasis>"
+        def admonition(self, text, kind):
+            if kind not in admonitions:
+                raise NotImplementedError(f"admonition {kind} not supported yet")
+            tag = admonitions[kind]
+            # we don't keep whitespace here because usually we'll contain only
+            # a single paragraph and the original docbook string is no longer
+            # available to restore the trailer.
+            return f"<{tag}><para>{text.rstrip()}</para></{tag}>"
+        def command(self, text):
+            return f"<command>{escape(text)}</command>"
+        def option(self, text):
+            return f"<option>{escape(text)}</option>"
+        def file(self, text):
+            return f"<filename>{escape(text)}</filename>"
+        def manpage(self, page, section):
+            title = f"<refentrytitle>{escape(page)}</refentrytitle>"
+            vol = f"<manvolnum>{escape(section)}</manvolnum>"
+            return f"<citerefentry>{title}{vol}</citerefentry>"
+
+        def finalize(self, data):
+            return "".join(data)
+
+    plugins = []
+
+    COMMAND_PATTERN = r'\{command\}`(.*?)`'
+    def command(md):
+        def parse(self, m, state):
+            return ('command', m.group(1))
+        md.inline.register_rule('command', COMMAND_PATTERN, parse)
+        md.inline.rules.append('command')
+    plugins.append(command)
+
+    FILE_PATTERN = r'\{file\}`(.*?)`'
+    def file(md):
+        def parse(self, m, state):
+            return ('file', m.group(1))
+        md.inline.register_rule('file', FILE_PATTERN, parse)
+        md.inline.rules.append('file')
+    plugins.append(file)
+
+    OPTION_PATTERN = r'\{option\}`(.*?)`'
+    def option(md):
+        def parse(self, m, state):
+            return ('option', m.group(1))
+        md.inline.register_rule('option', OPTION_PATTERN, parse)
+        md.inline.rules.append('option')
+    plugins.append(option)
+
+    MANPAGE_PATTERN = r'\{manpage\}`(.*?)\((.+?)\)`'
+    def manpage(md):
+        def parse(self, m, state):
+            return ('manpage', m.group(1), m.group(2))
+        md.inline.register_rule('manpage', MANPAGE_PATTERN, parse)
+        md.inline.rules.append('manpage')
+    plugins.append(manpage)
+
+    ADMONITION_PATTERN = re.compile(r'^::: \{([^\n]*?)\}\n(.*?)^:::\n', flags=re.MULTILINE|re.DOTALL)
+    def admonition(md):
+        def parse(self, m, state):
+            return {
+                'type': 'admonition',
+                'children': self.parse(m.group(2), state),
+                'params': [ m.group(1) ],
+            }
+        md.block.register_rule('admonition', ADMONITION_PATTERN, parse)
+        md.block.rules.append('admonition')
+    plugins.append(admonition)
+
+    def convertString(text: str) -> str:
+        rendered = mistune.markdown(text, renderer=Renderer(), plugins=plugins)
+        # keep trailing spaces so we can diff the generated XML to check for conversion bugs.
+        return rendered.rstrip() + text[len(text.rstrip()):]
+
+    def optionIs(option: Dict[str, Any], key: str, typ: str) -> bool:
+        if key not in option: return False
+        if type(option[key]) != dict: return False
+        if '_type' not in option[key]: return False
+        return option[key]['_type'] == typ
+
+    for (name, option) in options.items():
+        if optionIs(option, 'description', 'mdDoc'):
+            option['description'] = convertString(option['description']['text'])
+        if optionIs(option, 'example', 'literalMD'):
+            docbook = convertString(option['example']['text'])
+            option['example'] = { '_type': 'literalDocBook', 'text': docbook }
+        if optionIs(option, 'default', 'literalMD'):
+            docbook = convertString(option['default']['text'])
+            option['default'] = { '_type': 'literalDocBook', 'text': docbook }
+
+    return options
+
 warningsAreErrors = sys.argv[1] == "--warnings-are-errors"
 optOffset = 1 if warningsAreErrors else 0
 options = pivot(json.load(open(sys.argv[1 + optOffset], 'r')))
···
         file=sys.stderr)
     sys.exit(1)
 
-json.dump(unpivot(options), fp=sys.stdout)
+json.dump(convertMD(unpivot(options)), fp=sys.stdout)

nixos/modules/config/console.nix

···
       type = with types; either str path;
       default = "Lat2-Terminus16";
       example = "LatArCyrHeb-16";
-      description = ''
+      description = mdDoc ''
         The font used for the virtual consoles.  Leave empty to use
-        whatever the <command>setfont</command> program considers the
+        whatever the {command}`setfont` program considers the
         default font.
         Can be either a font name or a path to a PSF font file.
       '';

nixos/modules/config/debug-info.nix

···
     environment.enableDebugInfo = mkOption {
       type = types.bool;
       default = false;
-      description = ''
+      description = mdDoc ''
         Some NixOS packages provide debug symbols. However, these are
         not included in the system closure by default to save disk
         space. Enabling this option causes the debug symbols to appear
-        in <filename>/run/current-system/sw/lib/debug/.build-id</filename>,
-        where tools such as <command>gdb</command> can find them.
+        in {file}`/run/current-system/sw/lib/debug/.build-id`,
+        where tools such as {command}`gdb` can find them.
         If you need debug symbols for a package that doesn't
         provide them by default, you can enable them as follows:
-        <programlisting>
-        nixpkgs.config.packageOverrides = pkgs: {
-          hello = pkgs.hello.overrideAttrs (oldAttrs: {
-            separateDebugInfo = true;
-          });
-        };
-        </programlisting>
+
+            nixpkgs.config.packageOverrides = pkgs: {
+              hello = pkgs.hello.overrideAttrs (oldAttrs: {
+                separateDebugInfo = true;
+              });
+            };
       '';
     };
 

nixos/modules/misc/documentation.nix

···
       man.generateCaches = mkOption {
         type = types.bool;
         default = false;
-        description = ''
+        description = mdDoc ''
           Whether to generate the manual page index caches.
           This allows searching for a page or
-          keyword using utilities like
-          <citerefentry>
-            <refentrytitle>apropos</refentrytitle>
-            <manvolnum>1</manvolnum>
-          </citerefentry>
-          and the <literal>-k</literal> option of
-          <citerefentry>
-            <refentrytitle>man</refentrytitle>
-            <manvolnum>1</manvolnum>
-          </citerefentry>.
+          keyword using utilities like {manpage}`apropos(1)`
+          and the `-k` option of
+          {manpage}`man(1)`.
         '';
       };
 
···
       dev.enable = mkOption {
         type = types.bool;
         default = false;
-        description = ''
+        description = mdDoc ''
           Whether to install documentation targeted at developers.
-          <itemizedlist>
-          <listitem><para>This includes man pages targeted at developers if <option>documentation.man.enable</option> is
-                    set (this also includes "devman" outputs).</para></listitem>
-          <listitem><para>This includes info pages targeted at developers if <option>documentation.info.enable</option>
-                    is set (this also includes "devinfo" outputs).</para></listitem>
-          <listitem><para>This includes other pages targeted at developers if <option>documentation.doc.enable</option>
-                    is set (this also includes "devdoc" outputs).</para></listitem>
-          </itemizedlist>
+          * This includes man pages targeted at developers if {option}`documentation.man.enable` is
+            set (this also includes "devman" outputs).
+          * This includes info pages targeted at developers if {option}`documentation.info.enable`
+            is set (this also includes "devinfo" outputs).
+          * This includes other pages targeted at developers if {option}`documentation.doc.enable`
+            is set (this also includes "devdoc" outputs).
         '';
       };
 

nixos/modules/misc/man-db.nix

···
             ++ lib.optionals config.documentation.dev.enable [ "devman" ];
           ignoreCollisions = true;
         };
-        defaultText = lib.literalDocBook "all man pages in <option>config.environment.systemPackages</option>";
-        description = ''
-          The manual pages to generate caches for if <option>documentation.man.generateCaches</option>
+        defaultText = lib.literalMD "all man pages in {option}`config.environment.systemPackages`";
+        description = lib.mdDoc ''
+          The manual pages to generate caches for if {option}`documentation.man.generateCaches`
           is enabled. Must be a path to a directory with man pages under
-          <literal>/share/man</literal>; see the source for an example.
+          `/share/man`; see the source for an example.
           Advanced users can make this a content-addressed derivation to save a few rebuilds.
         '';
       };

nixos/modules/security/systemd-confinement.nix

···
       options.confinement.fullUnit = lib.mkOption {
         type = types.bool;
         default = false;
-        description = ''
+        description = lib.mdDoc ''
           Whether to include the full closure of the systemd unit file into the
           chroot, instead of just the dependencies for the executables.
 
-          <warning><para>While it may be tempting to just enable this option to
+          ::: {.warning}
+          While it may be tempting to just enable this option to
           make things work quickly, please be aware that this might add paths
           to the closure of the chroot that you didn't anticipate. It's better
-          to use <option>confinement.packages</option> to <emphasis
-          role="strong">explicitly</emphasis> add additional store paths to the
-          chroot.</para></warning>
+          to use {option}`confinement.packages` to **explicitly** add additional store paths to the
+          chroot.
+          :::
         '';
       };
 

nixos/modules/services/matrix/synapse.nix

···
 
       settings = mkOption {
         default = {};
-        description = ''
+        description = mdDoc ''
           The primary synapse configuration. See the
-          <link xlink:href="https://github.com/matrix-org/synapse/blob/v${cfg.package.version}/docs/sample_config.yaml">sample configuration</link>
+          [sample configuration](https://github.com/matrix-org/synapse/blob/v${cfg.package.version}/docs/sample_config.yaml)
           for possible values.
 
-          Secrets should be passed in by using the <literal>extraConfigFiles</literal> option.
+          Secrets should be passed in by using the `extraConfigFiles` option.
         '';
         type = with types; submodule {
           freeformType = format.type;
···
             registration_shared_secret = mkOption {
               type = types.nullOr types.str;
               default = null;
-              description = ''
+              description = mdDoc ''
                 If set, allows registration by anyone who also has the shared
                 secret, even if registration is otherwise disabled.
 
-                Secrets should be passed in via <literal>extraConfigFiles</literal>!
+                Secrets should be passed in via `extraConfigFiles`!
               '';
             };
 
             macaroon_secret_key = mkOption {
               type = types.nullOr types.str;
               default = null;
-              description = ''
+              description = mdDoc ''
                 Secret key for authentication tokens. If none is specified,
                 the registration_shared_secret is used, if one is given; otherwise,
                 a secret key is derived from the signing key.
 
-                Secrets should be passed in via <literal>extraConfigFiles</literal>!
+                Secrets should be passed in via `extraConfigFiles`!
               '';
             };
 
···
               example = literalExpression ''
                 config.services.coturn.static-auth-secret
               '';
-              description = ''
+              description = mdDoc ''
                 The shared secret used to compute passwords for the TURN server.
 
-                Secrets should be passed in via <literal>extraConfigFiles</literal>!
+                Secrets should be passed in via `extraConfigFiles`!
               '';
             };
 

nixos/modules/services/networking/mosquitto.nix

···
       hashedPassword = mkOption {
         type = uniq (nullOr str);
         default = null;
-        description = ''
+        description = mdDoc ''
           Specifies the hashed password for the MQTT User.
-          To generate hashed password install <literal>mosquitto</literal>
-          package and use <literal>mosquitto_passwd</literal>.
+          To generate hashed password install `mosquitto`
+          package and use `mosquitto_passwd`.
         '';
       };
 
···
         type = uniq (nullOr types.path);
         example = "/path/to/file";
         default = null;
-        description = ''
+        description = mdDoc ''
           Specifies the path to a file containing the
           hashed password for the MQTT user.
-          To generate hashed password install <literal>mosquitto</literal>
-          package and use <literal>mosquitto_passwd</literal>.
+          To generate hashed password install `mosquitto`
+          package and use `mosquitto_passwd`.
         '';
       };
 
···
     options = {
       plugin = mkOption {
         type = path;
-        description = ''
-          Plugin path to load, should be a <literal>.so</literal> file.
+        description = mdDoc ''
+          Plugin path to load, should be a `.so` file.
         '';
       };
 
       denySpecialChars = mkOption {
         type = bool;
-        description = ''
-          Automatically disallow all clients using <literal>#</literal>
-          or <literal>+</literal> in their name/id.
+        description = mdDoc ''
+          Automatically disallow all clients using `#`
+          or `+` in their name/id.
         '';
         default = true;
       };
 
       options = mkOption {
         type = attrsOf optionType;
-        description = ''
-          Options for the auth plugin. Each key turns into a <literal>auth_opt_*</literal>
+        description = mdDoc ''
+          Options for the auth plugin. Each key turns into a `auth_opt_*`
            line in the config.
         '';
         default = {};
···
 
       address = mkOption {
         type = nullOr str;
-        description = ''
-          Address to listen on. Listen on <literal>0.0.0.0</literal>/<literal>::</literal>
+        description = mdDoc ''
+          Address to listen on. Listen on `0.0.0.0`/`::`
           when unset.
         '';
         default = null;
···
 
       authPlugins = mkOption {
         type = listOf authPluginOptions;
-        description = ''
+        description = mdDoc ''
           Authentication plugin to attach to this listener.
-          Refer to the <link xlink:href="https://mosquitto.org/man/mosquitto-conf-5.html">
-          mosquitto.conf documentation</link> for details on authentication plugins.
+          Refer to the [mosquitto.conf documentation](https://mosquitto.org/man/mosquitto-conf-5.html)
+          for details on authentication plugins.
         '';
         default = [];
       };
···
 
     includeDirs = mkOption {
       type = listOf path;
-      description = ''
+      description = mdDoc ''
         Directories to be scanned for further config files to include.
         Directories will processed in the order given,
-        <literal>*.conf</literal> files in the directory will be
+        `*.conf` files in the directory will be
         read in case-sensistive alphabetical order.
       '';
       default = [];

nixos/modules/services/networking/syncthing.nix

···
       cert = mkOption {
         type = types.nullOr types.str;
         default = null;
-        description = ''
-          Path to the <literal>cert.pem</literal> file, which will be copied into Syncthing's
-          <link linkend="opt-services.syncthing.configDir">configDir</link>.
+        description = mdDoc ''
+          Path to the `cert.pem` file, which will be copied into Syncthing's
+          [configDir](#opt-services.syncthing.configDir).
         '';
       };
 
       key = mkOption {
         type = types.nullOr types.str;
         default = null;
-        description = ''
-          Path to the <literal>key.pem</literal> file, which will be copied into Syncthing's
-          <link linkend="opt-services.syncthing.configDir">configDir</link>.
+        description = mdDoc ''
+          Path to the `key.pem` file, which will be copied into Syncthing's
+          [configDir](#opt-services.syncthing.configDir).
         '';
       };
 
       overrideDevices = mkOption {
         type = types.bool;
         default = true;
-        description = ''
+        description = mdDoc ''
           Whether to delete the devices which are not configured via the
-          <link linkend="opt-services.syncthing.devices">devices</link> option.
-          If set to <literal>false</literal>, devices added via the web
+          [devices](#opt-services.syncthing.devices) option.
+          If set to `false`, devices added via the web
           interface will persist and will have to be deleted manually.
         '';
       };
 
       devices = mkOption {
         default = {};
-        description = ''
+        description = mdDoc ''
           Peers/devices which Syncthing should communicate with.
 
           Note that you can still add devices manually, but those changes
-          will be reverted on restart if <link linkend="opt-services.syncthing.overrideDevices">overrideDevices</link>
+          will be reverted on restart if [overrideDevices](#opt-services.syncthing.overrideDevices)
           is enabled.
         '';
         example = {
···
 
             id = mkOption {
               type = types.str;
-              description = ''
-                The device ID. See <link xlink:href="https://docs.syncthing.net/dev/device-ids.html"/>.
+              description = mdDoc ''
+                The device ID. See <https://docs.syncthing.net/dev/device-ids.html>.
               '';
             };
 
             introducer = mkOption {
               type = types.bool;
               default = false;
-              description = ''
+              description = mdDoc ''
                 Whether the device should act as an introducer and be allowed
                 to add folders on this computer.
-                See <link xlink:href="https://docs.syncthing.net/users/introducer.html"/>.
+                See <https://docs.syncthing.net/users/introducer.html>.
               '';
             };
 
             autoAcceptFolders = mkOption {
               type = types.bool;
               default = false;
-              description = ''
+              description = mdDoc ''
                 Automatically create or share folders that this device advertises at the default path.
-                See <link xlink:href="https://docs.syncthing.net/users/config.html?highlight=autoaccept#config-file-format"/>.
+                See <https://docs.syncthing.net/users/config.html?highlight=autoaccept#config-file-format>.
               '';
             };
 
···
       overrideFolders = mkOption {
         type = types.bool;
         default = true;
-        description = ''
+        description = mdDoc ''
           Whether to delete the folders which are not configured via the
-          <link linkend="opt-services.syncthing.folders">folders</link> option.
-          If set to <literal>false</literal>, folders added via the web
+          [folders](#opt-services.syncthing.folders) option.
+          If set to `false`, folders added via the web
           interface will persist and will have to be deleted manually.
         '';
       };
 
       folders = mkOption {
         default = {};
-        description = ''
+        description = mdDoc ''
           Folders which should be shared by Syncthing.
 
           Note that you can still add devices manually, but those changes
-          will be reverted on restart if <link linkend="opt-services.syncthing.overrideDevices">overrideDevices</link>
+          will be reverted on restart if [overrideDevices](#opt-services.syncthing.overrideDevices)
           is enabled.
         '';
         example = literalExpression ''
···
             devices = mkOption {
               type = types.listOf types.str;
               default = [];
-              description = ''
+              description = mdDoc ''
                 The devices this folder should be shared with. Each device must
-                be defined in the <link linkend="opt-services.syncthing.devices">devices</link> option.
+                be defined in the [devices](#opt-services.syncthing.devices) option.
               '';
             };
 
             versioning = mkOption {
               default = null;
-              description = ''
+              description = mdDoc ''
                 How to keep changed/deleted files with Syncthing.
                 There are 4 different types of versioning with different parameters.
-                See <link xlink:href="https://docs.syncthing.net/users/versioning.html"/>.
+                See <https://docs.syncthing.net/users/versioning.html>.
               '';
               example = literalExpression ''
                 [
···
                 options = {
                   type = mkOption {
                     type = enum [ "external" "simple" "staggered" "trashcan" ];
-                    description = ''
+                    description = mdDoc ''
                       The type of versioning.
-                      See <link xlink:href="https://docs.syncthing.net/users/versioning.html"/>.
+                      See <https://docs.syncthing.net/users/versioning.html>.
                     '';
                   };
                   params = mkOption {
                     type = attrsOf (either str path);
-                    description = ''
+                    description = mdDoc ''
                       The parameters for versioning. Structure depends on
-                      <link linkend="opt-services.syncthing.folders._name_.versioning.type">versioning.type</link>.
-                      See <link xlink:href="https://docs.syncthing.net/users/versioning.html"/>.
+                      [versioning.type](#opt-services.syncthing.folders._name_.versioning.type).
+                      See <https://docs.syncthing.net/users/versioning.html>.
                     '';
                   };
                 };
···
             ignoreDelete = mkOption {
               type = types.bool;
               default = false;
-              description = ''
+              description = mdDoc ''
                 Whether to skip deleting files that are deleted by peers.
-                See <link xlink:href="https://docs.syncthing.net/advanced/folder-ignoredelete.html"/>.
+                See <https://docs.syncthing.net/advanced/folder-ignoredelete.html>.
               '';
             };
           };
···
       extraOptions = mkOption {
         type = types.addCheck (pkgs.formats.json {}).type isAttrs;
         default = {};
-        description = ''
+        description = mdDoc ''
           Extra configuration options for Syncthing.
-          See <link xlink:href="https://docs.syncthing.net/users/config.html"/>.
+          See <https://docs.syncthing.net/users/config.html>.
         '';
         example = {
           options.localAnnounceEnabled = false;
···
         type = types.str;
         default = defaultUser;
         example = "yourUser";
-        description = ''
+        description = mdDoc ''
           The user to run Syncthing as.
-          By default, a user named <literal>${defaultUser}</literal> will be created.
+          By default, a user named `${defaultUser}` will be created.
         '';
       };
 
···
         type = types.str;
         default = defaultGroup;
         example = "yourGroup";
-        description = ''
+        description = mdDoc ''
           The group to run Syncthing under.
-          By default, a group named <literal>${defaultGroup}</literal> will be created.
+          By default, a group named `${defaultGroup}` will be created.
         '';
       };
 
···
         type = with types; nullOr str;
         default = null;
         example = "socks5://address.com:1234";
-        description = ''
+        description = mdDoc ''
           Overwrites the all_proxy environment variable for the Syncthing process to
           the given value. This is normally used to let Syncthing connect
           through a SOCKS5 proxy server.
-          See <link xlink:href="https://docs.syncthing.net/users/proxying.html"/>.
+          See <https://docs.syncthing.net/users/proxying.html>.
         '';
       };
 
···
           The path where the settings and keys will exist.
         '';
         default = cfg.dataDir + optionalString cond "/.config/syncthing";
-        defaultText = literalDocBook ''
-          <variablelist>
-            <varlistentry>
-              <term><literal>stateVersion >= 19.03</literal></term>
-              <listitem>
-                <programlisting>
-                  config.${opt.dataDir} + "/.config/syncthing"
-                </programlisting>
-              </listitem>
-            </varlistentry>
-            <varlistentry>
-              <term>otherwise</term>
-              <listitem>
-                <programlisting>
-                  config.${opt.dataDir}
-                </programlisting>
-              </listitem>
-            </varlistentry>
-          </variablelist>
+        defaultText = literalMD ''
+          * if `stateVersion >= 19.03`:
+
+                config.${opt.dataDir} + "/.config/syncthing"
+          * otherwise:
+
+                config.${opt.dataDir}
         '';
       };
 

nixos/modules/virtualisation/xen-dom0.nix

···
         default = false;
         type = types.bool;
         description =
-          ''
+          mdDoc ''
             Setting this option enables the Xen hypervisor, a
             virtualisation technology that allows multiple virtual
-            machines, known as <emphasis>domains</emphasis>, to run
+            machines, known as *domains*, to run
             concurrently on the physical machine.  NixOS runs as the
-            privileged <emphasis>Domain 0</emphasis>.  This option
+            privileged *Domain 0*.  This option
             requires a reboot to take effect.
           '';
       };