docs: Add rbtree documentation to the core-api

This file is close enough to being in rst format that I didn't feel
the need to alter it in any way.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Michel Lespinasse <walken@google.com>
Link: https://lore.kernel.org/r/20200401173343.17472-1-willy@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>