nvmecontrol: document power command

The description of the power command is missing. While the synopsis is
present, there's no explanation. Add one.

Reviewed by: mav, chuck
PR: 237866
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31122

(cherry picked from commit 445b5554bf97254a0ead3d70f801871d62dcfb62)