<https://www.freebsd.org/cgi/man.cgi?query=bectl&sektion=8&manpath=FreeBSD> defaults to FreeBSD 13.1-RELEASE but lacks the detail that's present when the page is viewed in a terminal on 13.1-RELEASE. man 8 bectl <https://github.com/freebsd/freebsd-src/commit/41e0b07a5e2a42cc2d6532ee529b6190bd14a69d?diff=split> (2022-04-06) was to releng/13.1.
I can't guess when, but it seems that this was fixed (thanks). The online page does now include Boot Environment Structures. For posterity: <https://web.archive.org/web/20220829120857/https://www.freebsd.org/cgi/man.cgi?query=bectl&sektion=8&manpath=FreeBSD>