Support `GET CMDDESC`

[Aram Peres]

https://networkupstools.org/docs/developer-guide.chunked/ar01s09.html

Form:
	GET CMDDESC <upsname> <cmdname>
	GET CMDDESC su700 load.on

Response:
	CMDDESC <upsname> <cmdname> "<description>"
	CMDDESC su700 load.on "Turn on the load immediately"

This is like DESC above, but it applies to the instant commands.