nixos: doc: optionally include all modules in manual generation

Before this change `man 5 configuration.nix` would only show options of modules in
the `baseModules` set, which consists only of the list of modules in
`nixos/modules/module-list.nix`

With this change applied and `documentation.nixos.includeAllModules` option enabled
all modules included in `configuration.nix` file will be used instead.

This makes configurations with custom modules self-documenting. It also means
that importing non-`baseModules` modules like `gce.nix` or `azure.nix`
will make their documentation available in `man 5 configuration.nix`.

`documentation.nixos.includeAllModules` is currently set to `false` by
default as enabling it usually uncovers bugs and prevents evaluation.
It should be set to `true` in a release or two.

This was originally implemented in #47177, edited for more configurability,
documented and rebased onto master by @oxij.

nixos/modules/misc/documentation.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, baseModules, ... }:
+{ config, lib, pkgs, baseModules, extraModules, modules, ... }:
 
 with lib;
 
@@ -6,6 +6,8 @@ let
 
   cfg = config.documentation;
 
+  manualModules = baseModules ++ optionals cfg.nixos.includeAllModules (extraModules ++ modules);
+
   /* For the purpose of generating docs, evaluate options with each derivation
     in `pkgs` (recursively) replaced by a fake with path "\${pkgs.attribute.path}".
     It isn't perfect, but it seems to cover a vast majority of use cases.
@@ -18,7 +20,7 @@ let
     options =
       let
         scrubbedEval = evalModules {
-          modules = [ { nixpkgs.localSystem = config.nixpkgs.localSystem; } ] ++ baseModules;
+          modules = [ { nixpkgs.localSystem = config.nixpkgs.localSystem; } ] ++ manualModules;
           args = (config._module.args) // { modules = [ ]; };
           specialArgs = { pkgs = scrubDerivations "pkgs" pkgs; };
         };
@@ -146,6 +148,17 @@ in
         '';
       };
 
+      nixos.includeAllModules = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether the generated NixOS's documentation should include documentation for all
+          the options from all the NixOS modules included in the current
+          <literal>configuration.nix</literal>. Disabling this will make the manual
+          generator to ignore options defined outside of <literal>baseModules</literal>.
+        '';
+      };
+
     };
 
   };