Motivation for this change

Previously if you set the freeform type (as introduced in #82743) to e.g. attrsOf (submodule ..), those submodule options wouldn't be shown in the manual.

Originally this problem was brought to my attention by @alexarice

With an option declaration like

{ lib, ... }: {
  options.foofoofoo = lib.mkOption {
    type = lib.types.submodule {
      freeformType = lib.types.attrsOf (lib.types.submodule {
        options.bar = lib.mkOption {
          description = "Freeform bar";
        };
      });
      options.qux.bar = lib.mkOption {
        description = "Qux bar";
      };
      options.bar = lib.mkOption {
        description = "Submodule bar";
      };
    };
  };
}

The manual now correctly shows

Ping @roberth

Things done

• Checked that the manual still builds
• Checked that even more complex cases like freeformType = attrsOf (attrsOf (submodule { freeformType = attrsOf ... work
• Added a test for checking that submodule options are present