Revise the text about the atomicity of the defined operations across
multiple processors.  In particular, clearly state that the operations
are always atomic when they are applied to the default memory type
that is used by the kernel (and applications).

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.

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.