documentation: Fix control dependency and identical stores

The summary of the "CONTROL DEPENDENCIES" section incorrectly states that
barrier() may be used to prevent compiler reordering when more than one
leg of the control-dependent "if" statement start with identical stores.
This is incorrect at high optimization levels.  This commit therefore
updates the summary to match the detailed description.

Reported by: Jianyu Zhan <nasa4836@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/memory-barriers.txt

@@ -800,9 +800,13 @@ In summary:
       use smp_rmb(), smp_wmb(), or, in the case of prior stores and
       later loads, smp_mb().
 
-  (*) If both legs of the "if" statement begin with identical stores
-      to the same variable, a barrier() statement is required at the
-      beginning of each leg of the "if" statement.
+  (*) If both legs of the "if" statement begin with identical stores to
+      the same variable, then those stores must be ordered, either by
+      preceding both of them with smp_mb() or by using smp_store_release()
+      to carry out the stores.  Please note that it is -not- sufficient
+      to use barrier() at beginning of each leg of the "if" statement,
+      as optimizing compilers do not necessarily respect barrier()
+      in this case.
 
   (*) Control dependencies require at least one run-time conditional
       between the prior load and the subsequent store, and this