Martin,
I feel your pain! My usual approach is Google (which searches this forum), then IRC/Twitter, and then opening up the code and stepping through it with a debugger. You'd be surprised how much you can learn by doing that.
Recently I've discovered the architecture of MODX doesn't lend itself to being thought of as a traditional API: processors function very much as API method could would (think of them as a remote REST service) but the core, for all it's layered, is still tightly coupled (e.g. processors return ExtJS JSON on success, and there appears to be no way to remove this Ext-specific layer and use the processor as a 'pure' PHP API call).
There are a small number of people who know MODX very well, and a much larger number of people who use it on the surface (and usually dabble in PHP) yet think they know the answer. It does make getting help frustrating!
Good luck with getting your Extra finished