mount: document atime option

atime is enabled by default, but may be specified explicitly so that any
future change in the default would not have an effect on a given mount.

Reviewed by: olce, rgrimes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43580

(cherry picked from commit 54d31cfb278982ff8c2af97a13e40b1a3ebd838e)