Daniel Vetter of Intel has a big DRM pull request for drm-next (to land in Linux 3.15) that updates the subsystem's documentation.
Daniel wrote on the mailing list, "Here's my drm documentation update and driver api polish pull request. Alex reviewed the entire pile, I've applied a little bit of spelling polish in a few places since then and otherwise the Usual Suspects (David, Rob, ...) don't seem up to have another look at it (I've poked them on irc). So I think it's as good as it gets ;-)"
Lack of adequate documentation and its high barrier to entry continues to be one of the most often heard complaints for why more open-source developers don't get involved in the development of the open-source Linux graphics drivers. For those open-source fans looking to get involved, there's the Mesa Newbie Projects list that walks inexperienced GPU driver developers through adding some basic OpenGL extensions to Mesa. There's also support and other options available via the Wikis, IRC channels, or even our forums. Crowd-funding has even worked for getting involved with Mesa and making OpenGL driver improvements.