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.
