default merge source is confusing (docs mismatch behaviour, behaviour is complex)

bzr merge --help claims it merges from the parent, or a --remembered url; the actual behaviour seems to be from submit, then parent. Or something - i haven't checked the code.

I know this has changed recently, and flopped around a bit; it would be good to find some basic primitive to build all this stuff out of that would be less confusing - particularly as submit branches seem to be somewhat fading *anyway* with the stronger focus on API usage and explicit metadata in plugins to meet individual workflows.