refcount.9: Update refcount_acquire(9) description after a67d540832b8

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D36943

(cherry picked from commit 87fa64c569c54c2ec444ada699acde2ada696d97)