[MERGE] Clarify use of underscores in HACKING

[Martin Pool]

Ian's patch is a reasonable concise summary, I have no objection to it
going in.  I think you could usefully add some more text from John's
mail, which is a good summary.

Given that we have really only one bit to play with (if we're going to
mostly avoid __goo or other conventions) and that there's a scale of
five levels, I think "private to bzrlib" or not is the important place
to draw the line.  In case (c), if another bzrlib class uses something
that's meant to be private, that might mean more code to update later,
but it can in principle be updated whenever someone changes the class.
 That's not true for plugin code - we can't grep all of it to see what
broke - so it's the most important place to give a clear interface to.
 So I'm happy using _ in more or less what it currently means.

I guess we could use the opposite of deprecated_ methods to mark
things that are guaranteed public.  I don't think that would be
worthwhile just yet.

What might help a lot is when we get buildbot running to have it test
bzr.dev against all known plugins.

-- 
Martin