rbtree: add comment to rb_insert_augmented()

The comment is copied from Documentation/rbtree.txt, but this comment is
so important that it should also be in the code.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>