Documentation/memory-barriers.txt: Add long atomic examples to memory-barriers.txt

Although the atomic_long_t functions are quite useful, they are
a bit obscure.  This commit therefore adds the common ones
alongside their atomic_t counterparts in
Documentation/memory-barriers.txt.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <linux-arch@vger.kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1386799151-2219-2-git-send-email-paulmck@linux.vnet.ibm.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>