documentation: Add alternative release-acquire outcome

The memory-barriers.txt discussion of local transitivity and
release-acquire chains leaves out discussion of the outcome of
the read from "u".  This commit therefore adds an outcome showing
that you can get a "1" from this read even if the release-acquire
pairs don't line up.
Reported-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/memory-barriers.txt

@@ -1372,6 +1372,10 @@ is possible:
 
 	r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
 
+As an aside, the following outcome is also possible:
+
+	r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1
+
 Although cpu0(), cpu1(), and cpu2() will see their respective reads and
 writes in order, CPUs not involved in the release-acquire chain might
 well disagree on the order.  This disagreement stems from the fact that