documentation: RCU grace-period memory ordering guarantees

This commit provides text and diagrams showing how Tree RCU implements
its grace-period memory ordering guarantees.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>