bectl: document the -r flag to bectl

Independent of all of the commands, bectl itself takes an -r flag that
specifies the BE root to use. This was originally added to facilitate
testing, but it was later discovered to be incredibly useful in other
scenarios; e.g., trying to recover some boot environments in rescue
media.

The "BE root" described here is the parent dataset that holds boot
environments, but I've no idea if that's an accepted definition for that
dataset.

Reviewed by: gallatin, imp, Pau Amma
MFC after: 1 week
Differential Review: https://reviews.freebsd.org/D39710