lib/types: Make submodules use the freeform type description

Submodules that have a freeform type set behave as if that was the type
of the option itself (for values that don't have an option). Since the
submodules options are shown as separate entries in the manual, it makes
sense to show the freeform type as the submodule type.

lib/types.nix

@@ -486,9 +486,15 @@ rec {
           else value
         ) defs;
 
+        freeformType = (evalModules {
+          inherit modules specialArgs;
+          args.name = "‹name›";
+        })._module.freeformType;
+
       in
       mkOptionType rec {
         name = "submodule";
+        description = freeformType.description or name;
         check = x: isAttrs x || isFunction x || path.check x;
         merge = loc: defs:
           (evalModules {