The attached patch appears to work correctly and includes a test for pydoc (text output). There could be more testing (HTML output, annotations, positional-only arguments).

With this patch I can get usable documentation for Cocoa classes with a patched version of PyObjC that includes an __signature__ property on ObjC method objects.

As mentioned before I'm not sure about the 'render' method added to inspect.Signature and inspect.Parameter. These methods were needed to generate output that's compatible with the current pydoc output, in particular to emit better HTML output.