freebsd-update: Clarify help text

Improve help text to include example release numbers for reference
and clarify the -F option.

PR: 231185, 214619
Submitted by: Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by: delphij, rgrimes
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18944