memory-barriers: Replace uses of "transitive"

The current version of memory-barriers.txt misuses the term "transitive",
so this commit replaces it with multi-copy atomic, also adding a
definition of this term.
Reported-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>