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