documentation: Transitivity is not cumulativity

The "transitivity" section mentions cumulativity in a potentially
confusing way.  Contrary to the current wording, cumulativity is
not transitivity, but rather a hardware discipline that can be used
to implement transitivity on ARM and PowerPC CPUs.  This commit
therefore deletes the mention of cumulativity.

Reported-by: Luc Maranget <luc.maranget@inria.fr>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/memory-barriers.txt

@@ -1270,7 +1270,7 @@ TRANSITIVITY
 
 Transitivity is a deeply intuitive notion about ordering that is not
 always provided by real computer systems.  The following example
-demonstrates transitivity (also called "cumulativity"):
+demonstrates transitivity:
 
 	CPU 1			CPU 2			CPU 3
 	=======================	=======================	=======================