bectl: add some discussion about boot environment layouts

bectl: add some discussion about boot environment layouts

Discuss the standard type of layout, as well as the "deep" BE layout,
and some of the properties of both.  Point the various -r flags at this
new section, to help users understand which they're working with and
what the -r flag is actually doing. Note that we may just deprecate the
-r flag in future versions, but the flag will be recognized as a NOP at
that point.

Reviewed by:	pauamma_gundo.com, rew
Differential Revision:	https://reviews.freebsd.org/D34538