Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()

The description of smp_mb__before_atomic() and smp_mb__after_atomic()
in Documentation/atomic_t.txt is slightly terse and misleading.  It
does not clearly state which other instructions are ordered by these
barriers.

This improves the text to make the actual ordering implications clear,
and also to explain how these barriers differ from a RELEASE or
ACQUIRE ordering.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>