documentation: No acquire/release for RCU readers

Documentation/memory-barriers.txt calls out RCU as one of the sets
of primitives associated with ACQUIRE and RELEASE.  There really
is an association in that rcu_assign_pointer() includes a RELEASE
operation, but a quick read can convince people that rcu_read_lock() and
rcu_read_unlock() have ACQUIRE and RELEASE semantics, which they do not.

This commit therefore removes RCU from this list in order to avoid
this confusion.

Reported-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>