I would absolutely love to do this! I'm just not sure where to start(browse github?) or how to approach a team (re: introvert). I hate to look like the sad-eyed puppy waiting for a bone...
GitHub would be a good place to start, but really it should be a project you've used a fair amount and that you really like.
No need to approach them. If you want to fill in the gaps in the docs they have, then fork the project, improve the documentation, and send them a pull request. If you want to start a separate project documenting the project, whip something up that's decent but doesn't take you too long and send them a GitHub message.
Don't be shy. The sense I get is that people that want to do documentation are extremely hard to come by, so most OSS project leaders would jump for joy if they got a message about someone improving documentation for their project.
If you're just starting out, it might be easier for you to join an existing team and learn from the more experienced members. The Fedora Project is geared toward contributors and is very friendly towards new ones especially. You usually just have to send an email to the list of the team you want to participate in or drop by the IRC channel. Someone will then start you working on whatever you want to do.
