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

[emaste]

Summary

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

[grahamperrin]

English looks good to me.

Additionally, consider this D42022 as a fix for:

• 263290 – freebsd-update(8) should create new /var/db/freebsd-update dir

The phrase under (a directory) may be enough for readers of the manual page to understand that it's not appropriate to delete the directory itself.

[bcr]

OK from the docs side of things.