kokr/memory-barriers.txt: Apply atomic_t.txt change

This commit applies memory-barriers.txt part of upstream change, commit
706eeb3e ("Documentation/locking/atomic: Add documents for new
atomic_t APIs") to Korean translation.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>