Documentation: Memory barrier semantics of atomic_xchg()

Add atomic_xchg() to documentation for atomic operations and
memory barriers.

Signed-off-by: Richard Braun <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>

Documentation/atomic_ops.txt

···
 the given new value.  It returns the old value that the atomic variable v had
 just before the operation.
 
+atomic_xchg requires explicit memory barriers around the operation.
+
 	int atomic_cmpxchg(atomic_t *v, int old, int new);
 
 This performs an atomic compare exchange operation on the atomic value v,

Documentation/memory-barriers.txt

···
 
 	xchg();
 	cmpxchg();
+	atomic_xchg();
 	atomic_cmpxchg();
 	atomic_inc_return();
 	atomic_dec_return();