Add comments about the module system interface

Ideally the module system could be configured pretty much completely by
the contents of the modules themselves, so add comments about avoiding
complicating it further and possibly removing now-redundant
configurability from the existing interface.

lib/types.nix

@@ -200,7 +200,12 @@ in rec {
           let
             coerce = def: if isFunction def then def else { config = def; };
             modules = opts' ++ map (def: { _file = def.file; imports = [(coerce def.value)]; }) defs;
-          in (evalModules { inherit modules; args.name = last loc; prefix = loc; }).config;
+          in (evalModules {
+            inherit modules;
+            # !!! See comment about args in lib/modules.nix
+            args.name = last loc;
+            prefix = loc;
+          }).config;
         getSubOptions = prefix: (evalModules
           { modules = opts'; inherit prefix;
             # FIXME: hack to get shit to evaluate.