nixos/acme: improve documentation

* Use literalExample for better readability
* Clarify a bit wrt. 'webroot' and 'allowKeysForGroup'

nixos/modules/security/acme.nix

···
         description = ''
           Where the webroot of the HTTP vhost is located.
           <filename>.well-known/acme-challenge/</filename> directory
-          will be created automatically if it doesn't exist.
+          will be created below the webroot if it doesn't exist.
           <literal>http://example.org/.well-known/acme-challenge/</literal> must also
           be available (notice unencrypted HTTP).
         '';
···
       allowKeysForGroup = mkOption {
         type = types.bool;
         default = false;
-        description = "Give read permissions to the specified group to read SSL private certificates.";
+        description = ''
+          Give read permissions to the specified group
+          (<option>security.acme.group</option>) to read SSL private certificates.
+        '';
       };
 
       postRun = mkOption {
···
       extraDomains = mkOption {
         type = types.attrsOf (types.nullOr types.str);
         default = {};
-        example = {
-          "example.org" = "/srv/http/nginx";
-          "mydomain.org" = null;
-        };
+        example = literalExample ''
+          {
+            "example.org" = "/srv/http/nginx";
+            "mydomain.org" = null;
+          }
+        '';
         description = ''
           Extra domain names for which certificates are to be issued, with their
           own server roots if needed.
···
         description = ''
           Attribute set of certificates to get signed and renewed.
         '';
-        example = {
-          "example.com" = {
-            webroot = "/var/www/challenges/";
-            email = "[e-mail]";
-            extraDomains = { "www.example.com" = null; "foo.example.com" = "/var/www/foo/"; };
-          };
-          "bar.example.com" = {
-            webroot = "/var/www/challenges/";
-            email = "[e-mail]";
-          };
-        };
+        example = literalExample ''
+          {
+            "example.com" = {
+              webroot = "/var/www/challenges/";
+              email = "[e-mail]";
+              extraDomains = { "www.example.com" = null; "foo.example.com" = "/var/www/foo/"; };
+            };
+            "bar.example.com" = {
+              webroot = "/var/www/challenges/";
+              email = "[e-mail]";
+            };
+          }
+        '';
       };
     };
   };