modules/context: init with isDocs

Replaced the `isDocs` specialArg added in #1807 with an internal module
option.

We should only use `specialArgs` when absolutely necessary, a module or
`_module.args` is preferred, but those aren't available until `config`
is evaluated so they can't be used for `imports`.

The main problem with `specialArgs` is it makes our modules less
portable.

Luckily that shouldn't be an issue for these context flags.

modules/top-level/files/default.nix

@@ -3,12 +3,10 @@
   config,
   lib,
   helpers,
-  specialArgs,
   ...
 }:
 let
   inherit (lib) types;
-  isDocs = specialArgs.isDocs or false;
 
   fileModuleType = types.submoduleWith {
     shorthandOnlyDefinesConfig = true;
@@ -17,7 +15,7 @@ let
       defaultPkgs = pkgs;
     };
     # Don't include the modules in the docs, as that'd be redundant
-    modules = lib.optionals (!isDocs) [
+    modules = lib.optionals (!config.isDocs) [
       ../../.
       ./submodule.nix
     ];