MFC r245012:

Document the output of the show command. Modified version of patch
provided by Bas Smeelen <b.smeelen@ose.nl>. Use of 'gpart list'
suggested by by Andrey V. Elsukov <ae@FreeBSD.org>.

PR: docs/174270
Submitted by: Ronald F.Guilmette <rfg@tristatelogic.com>
Reviewed by: ae (block sizes)