I don't think that's true. Watch the pgsql-docs mailing list and you will see a steady stream of people pointing issues in the docs.
Moreover, there is a steady stream of new features. New features that need polish and docs. I've got a patch in the mailing list (maybe, hopefully, knock on wood) to add the first code samples to demonstrate Table Access Methods.
For that matter, I don't think there has yet been committed an a second /dev/null or in-memory Table Access Method. Though I'm aware of one person's effort to add a /dev/null Table Access Method. But I don't think this has been merged yet.
This is unrelated to what Robert is talking about though. I don't love the contribution process. But there is definitely a lot to contribute to if you do spend time and keep an a eye out for where improvement might be useful/accepted.