PyPy is quite friendly to new contributors, so if you want to help speed up JSON or improve docs, visit #pypy on freenode and offer to pitch in, and I'm sure some core developers will do their best to help you help them.
Or if you really want a feature but don't have time to work on it, see if any PyPy devs are accepting contract work. I suspect speeding up JSON wouldn't take one of the experts long, so it probably wouldn't cost you much.
Docs are more of a moving target: no matter how much you write, users will always want more (and then some of those complaining users will fail to actually read what you do write). There was a tutorial session at PyCon US this year about using PyPy to speed up Python code, and the video is on Youtube.