bectl(8): sync man page and help text

Sync man page with behavior of bectl(8).

Sync help text with man page.

PR: 246697
Reported by: olgeni
Submitted by: olgeni (with changes)
Reviewed by: kevans, olgeni
Approved by: kevans (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27482