freebsd-update: add a note about when files may be deleted

Files under /var/db/freebsd-update are required during the upgrade
process, and to support rollback. They may be deleted if no upgrade is
in progress and rollback will not be required.

PR: 273601
Reviewed by: bcr
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42022

(cherry picked from commit 30beebe444f5f415a388be9df91107246bd32674)
(cherry picked from commit ffe4456d7d9f2c6bb14e9b747766adc3042f0b9e)