Documentation/memory-barriers.txt: Conditional must use prior load

A control dependency consists of a load, a conditional that depends on
that load, and a store.  This commit emphasizes this point in the
summary.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Documentation/memory-barriers.txt

@@ -724,7 +724,8 @@ In summary:
       later loads, smp_mb().
 
   (*) Control dependencies require at least one run-time conditional
-      between the prior load and the subsequent store.  If the compiler
+      between the prior load and the subsequent store, and this
+      conditional must involve the prior load.  If the compiler
       is able to optimize the conditional away, it will have also
       optimized away the ordering.  Careful use of ACCESS_ONCE() can
       help to preserve the needed conditional.