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>