[PATCH] Yet another RCU documentation update

Update RCU documentation based on discussions and review of RCU-based tree
patches.  Add an introductory whatisRCU.txt file.

Signed-off-by:  <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>