memory-barriers.txt: Sort wait_event* and wait_on_bit* list alphabetically

Commit 88172700423c ("docs/memory-barriers.txt: Add wait_event_cmd()
and wait_event_exclusive_cmd()") added two APIs without taking care
of the list order.  Sort the list for readability.

While there, make it clear that this is incomplete by saying
"for example".

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>