rbtree: add some additional comments for rebalancing cases

While overall the code is very nicely commented, it might not be
immediately obvious from the diagrams what is going on.  Add a very
brief summary of each case.  Opposite cases where the node is the left
child are left untouched.

Link: http://lkml.kernel.org/r/20170719014603.19029-4-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>