documentation: Record rcu_dereference() value mishandling

Recent LKML discussings (see http://lwn.net/Articles/586838/ and
http://lwn.net/Articles/588300/

 for the LWN writeups) brought out
some ways of misusing the return value from rcu_dereference() that
are not necessarily completely intuitive.  This commit therefore
documents what can and cannot safely be done with these values.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>