They've been saying it'll come eventually. There have been a few people who ran it through PHPDoc to get a 'manual', but other than that there isn't anything 'substantial' in that regard. The code is relatively self explanatory, but for someone who is just learning PHP like you (please don't take offense to that, I got that vibe and slightly remember you saying that) it can be quite confusing. The developers that actively contribute are almost always willing to help you through learning it.
Post questions away and you'll learn more than a document can usually teach you.
One of the best things to do when learning to develop for XenForo is to read up where XenForo might be doing something similar. You can learn a lot from how the official code works and the comments on it, and quite often be shown exactly how to do something well. And of course there's always asking here, which I've found to work pretty consistently