tzsetup: add detail for -s option

Skipping the UTC question via -s will not create or delete
/etc/wall_cmos_clock.

Reported by: Tomoaki AOKI
Reviewed by: imp, allanjude, jrm
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45576

(cherry picked from commit 70e3e1bde9bf0ddf7fc18b97487fe2d23d4fc5ed)