atomic.9: fix description of acquire and release

The ordering point is not the atomic operation itself, but the load for
acquire or store for release done as part of the atomic. This does not
matter for atomic_load_acq and atomic_store_rel, but does matter for RWM
operations.

Noted by: alc
Reviewed by: markj
Discussed with: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D52744