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.

docs/mdbook/default.nix

@@ -14,7 +14,6 @@ let
     specialArgs = {
       inherit helpers;
       defaultPkgs = pkgs;
-      isDocs = true;
     };
   };