OpenMAX IL is a standard, so the first place to look is khronos.org.
We do also have some html documentation generated from comments in the source which describes some of the extensions used.
I'll try to get that tidied up and published.
But really, OpenMax IL is hard to use. You'd be well advised to look very closely at the XBMC code.
is another place to look. (Video playback from python).