atomics: update documentation

atomics: update documentation

Some of the constraints on operand sizes have been relaxed, so adjust the
documentation.

Deprecate atomic_mb_read and atomic_mb_set; it is not really possible to
use them correctly because they do not interoperate with sequentially-consistent
RMW operations.

Finally, extend the memory barrier pairing section to cover acquire and
release semantics in general, roughly based on the KVM Forum 2016 talk,
"<atomic.h> weapons".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>