doc: Rewrite confusing statement about memory barriers

The "Write (or store) memory barriers" bullet of the "Variety of memory
barriers" section, calls out a sequential order of stores, which is
confusing since sequential ordering is not guaranteed.

This commit therefore rewords to avoid mentioning a sequence of stores
to clarify the intent.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>