Probably either in the chapter synopsis or a section just after that, with references elsewhere in the chapter as needed or useful.
See also: bug 263303 comment 0 concerning automated use of bectl(8) by freebsd-update(8)
(In reply to Graham Perrin from comment #1) Thanks. Specifically, include a mention of duplicating what https://cgit.freebsd.org/src/commit/usr.sbin/freebsd-update?id=f28f138905416c45ebaa6429f44a0b88a72f54b1 and https://cgit.freebsd.org/src/commit/usr.sbin/freebsd-update?id=e01e8f911b935eabcc35b4d121951e4e21042ee5 (both MFC'd to 13-stable and present in 13.1) can do automatically. Is that what you meant?
(In reply to PauAmma from comment #2) I opened a review for the intended release note: <https://reviews.freebsd.org/D35213>