documentation: Add acquire/release barriers to pairing rules

It is possible to pair acquire and release barriers with other barriers,
so this commit adds them to the list in the SMP barrier pairing section.

Reported-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Updated pairing discussion as suggested by Peter Zijlstra. ]

diff --cc Documentation/memory-barriers.txt
Simple merge