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/default.nix

@@ -58,7 +58,10 @@ let
       ) opt.declarations;
     };
 
-  modules = [ ../modules/top-level ];
+  modules = [
+    ../modules/top-level
+    { isDocs = true; }
+  ];
 
   hmOptions = builtins.removeAttrs (lib.evalModules {
     modules = [ (import ../wrappers/modules/hm.nix { inherit lib; }) ];
@@ -73,7 +76,6 @@ rec {
           specialArgs = {
             inherit helpers;
             defaultPkgs = pkgsDoc;
-            isDocs = true;
           };
         })
         options

docs/mdbook/default.nix

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