Stop describing an acquire operation as a read barrier and a release

Stop describing an acquire operation as a read barrier and a release
operation as a write barrier.  That description has never been correct,
and it has caused confusion.  An acquire operation orders writes as well
as reads, and a release operation orders reads as well as writes.

Also, explicitly say that a thread doesn't see its own accesses being
reordered.  The reordering of a thread's accesses is only (potentially)
visible to another thread.  Thus, memory barriers need only be used to
control the ordering of accesses between threads, not within a thread.

Reviewed by:	bde, kib
Discussed with:	jhb
MFC after:	1 week