AFAK, koen said it will be in the next versions of GPD. But for now you can edit your plugin's plugin.xml file directly without PDE editors. Look original GPD's plugin.xml file for examples.
The only current documentation is indeed the code... But as I already said a couple of times in other forum entries, the extension points will probably change significantly in the next generation of the GPD.
I will post some ideas about this next generation shortly.
expect a quick birth of the next generation! Thank your very much.