You can’t document something until it’s in a stable state.
bbPress 1.0 is twice the size of bbPress 0.9 and constantly changing, even in the alpha.
Database functions have been mostly the same except the entire meta changed.
But the fundamentals are identical to how WordPress works so you can use the WP codex for that.
Oh and WordPress didn’t have decent documentation until many years after it was around, don’t expect bbPress to be any different. Unless someone volunteers for free, Matt is unlikely to waste paid employee time on documentation.