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/misc/default.nix

@@ -6,6 +6,7 @@ let
 in
 {
   imports = [
+    ./context.nix
     ./nixpkgs.nix
     ./nixvim-info.nix
     (nixosModules + "/misc/assertions.nix")