fixup! doc: Add Module System chapter start

doc/module-system/module-system.chapter.md

···
 
 Compared to plain Nix, it adds documentation, type checking and composition or extensibility.
 
-NOTE: This chapter is new and not complete yet. For a gentle introduction to the module system, in the context of NixOS, see [Writing NixOS Modules](https://nixos.org/manual/nixos/unstable/index.html#sec-writing-modules) in the NixOS manual.
+::: {.note}
+This chapter is new and not complete yet. For a gentle introduction to the module system, in the context of NixOS, see [Writing NixOS Modules](https://nixos.org/manual/nixos/unstable/index.html#sec-writing-modules) in the NixOS manual.
+:::
+
 
 ## `lib.evalModules` {#module-system-lib-evalModules}
 
-Evaluate a set of modules.  The result is a set with the attributes:
+Evaluate a set of modules. This function is typically only used once per application (e.g. once in NixOS, once in Home Manager, ...).
 
 ### Parameters {#module-system-lib-evalModules-parameters}
 
 #### `modules` {#module-system-lib-evalModules-param-modules}
 
-A list of modules. These are merged together using various methods to form the final configuration.
+A list of modules. These are merged together to form the final configuration.
+<!-- TODO link to section about merging, TBD -->
 
 #### `specialArgs` {#module-system-lib-evalModules-param-specialArgs}
 
 An attribute set of module arguments that can be used in `imports`.
 
-This is in contrast to `config._module.args`, which is only available within the module fixpoint, which does not exist before all imports are resolved.
+This is in contrast to `config._module.args`, which is only available after all `imports` have been resolved.
 
 #### `specialArgs.class` {#module-system-lib-evalModules-param-specialArgs-class}
 
-If the `class` attribute is set in `specialArgs`, the module system will rejected modules with a different `class`.
+If the `class` attribute is set in `specialArgs`, the module system will reject modules with a different `class`.
 
-This improves the error message that users will encounter when they import an incompatible module that was designed for a different class of configurations.
+The `class` value should be in lower [camel case](https://en.wikipedia.org/wiki/Camel_case).
 
-The `class` value should be in camelcase, and, if applicable, it should match the prefix of the attributes used in (experimental) flakes. Some examples are:
+If applicable, the `class` should match the "prefix" of the attributes used in (experimental) [flakes](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#description). Some examples are:
 
- - `nixos`: NixOS modules
+ - `nixos` as in `flake.nixosModules`
  - `nixosTest`: modules that constitute a [NixOS VM test](https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests)
+<!-- We've only just started with `class`. You're invited to add a few more. -->
 
 #### `prefix` {#module-system-lib-evalModules-param-prefix}
 
 A list of strings representing the location at or below which all options are evaluated. This is used by `types.submodule` to improve error reporting and find the implicit `name` module argument.
 
 ### Return value {#module-system-lib-evalModules-return-value}
+
+The result is an attribute set with the following attributes:
 
 #### `options` {#module-system-lib-evalModules-return-value-options}
 
-The nested set of all option declarations.
+The nested attribute set of all option declarations.
 
 #### `config` {#module-system-lib-evalModules-return-value-config}
 
-The nested set of all option values.
+The nested attribute set of all option values.
 
 #### `type` {#module-system-lib-evalModules-return-value-type}
 
-A module system type representing the module set as a submodule, to be extended by configuration from the containing module set.
+A module system type. This type is an instance of `types.submoduleWith` containing the current [`modules`](#module-system-lib-evalModules-param-modules).
 
-This is also available as the module argument `moduleType`.
+The option definitions that are typed with this type will extend the current set of modules, like [`extendModules`](#module-system-lib-evalModules-return-value-extendModules).
+
+However, the value returned from the type is just the [`config`](#module-system-lib-evalModules-return-value-config), like any submodule.
+
+This type is also available to the [`modules`](#module-system-lib-evalModules-param-modules) as the module argument `moduleType`.
 
 #### `extendModules` {#module-system-lib-evalModules-return-value-extendModules}
 
-A function similar to `evalModules` but building on top of the module set. Its arguments, `modules` and `specialArgs` are added to the existing values.
+A function similar to `evalModules` but building on top of the already passed [`modules`](#module-system-lib-evalModules-param-modules). Its arguments, `modules` and `specialArgs` are added to the existing values.
 
-Using `extendModules` a few times has no performance impact as long as you only reference the final `options` and `config`.
-If you do reference multiple `config` (or `options`) from before and after `extendModules`, performance is the same as with multiple `evalModules` invocations, because the new modules' ability to override existing configuration fundamentally requires a new fixpoint to be constructed.
+The real work of module evaluation happens while computing the values in `config` and `options`, so multiple invocations of `extendModules` have a particularly small cost, as long as only the final `config` and `options` are evaluated.
+
+If you do reference multiple `config` (or `options`) from before and after `extendModules`, evaluation performance is the same as with multiple `evalModules` invocations, because the new modules' ability to override existing configuration fundamentally requires constructing a new `config` and `options` fixpoint.
 
-This is also available as a module argument.
+This functionality is also available to modules as the `extendModules` module argument.
 
 #### `_module` {#module-system-lib-evalModules-return-value-_module}
 
 A portion of the configuration tree which is elided from `config`. It contains some values that are mostly internal to the module system implementation.
+
+<!-- TODO: when markdown migration is complete, make _module docs visible again and reference _module docs. Maybe move those docs into this chapter? -->