freebsd-update.8: Document CreateBootEnv

Also, add bectl(8) to section "See Also". [1]

PR: 261716
Reviewed by: debdrup, pauamma_gundo.com
MFC after: 1 week
Fixes: f28f13890541 freebsd-update: create a ZFS boot environment on install
Differential Revision: https://reviews.freebsd.org/D34169
Co-authored-by: Tobias Rehbein <tobias.rehbein@web.de> [1]

(cherry picked from commit 6d17f2d04302af6408127b9f8a307e2977c6d086)