The above currently does not exist, leaving hapless users to attempt to figure it out themselves: https://www.reddit.com/r/freebsd/comments/fg7598/pkg_removes_firefox_on_update_help_freebsd/fkagx8m/. Also the instructions in /etc/pkg/FreeBSD.conf are unclear. They appear to indicate the user should create a separate file elsewhere and edit that, but editing it directly apparently works too.
(In reply to jdrch from comment #0)
Yes, it is documented, here (section 4.4.2.):
Not sure how none of us on the Reddit thread noticed that. Thanks for pointing out. This can be closed, if it isn't already.