doc/atomic_ops: Clarify smp_mb__{before,after}_atomic()

This commit explicitly states that surrounding a non-value-returning
atomic read-modify atomic operations provides full ordering, just as
is provided by value-returning atomic read-modify-write operations.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/core-api/atomic_ops.rst

@@ -303,6 +303,11 @@ defined which accomplish this::
 	void smp_mb__before_atomic(void);
 	void smp_mb__after_atomic(void);
 
+Preceding a non-value-returning read-modify-write atomic operation with
+smp_mb__before_atomic() and following it with smp_mb__after_atomic()
+provides the same full ordering that is provided by value-returning
+read-modify-write atomic operations.
+
 For example, smp_mb__before_atomic() can be used like so::
 
 	obj->dead = 1;