mmexternal docs lack important information

[timbunce]

The code says "Note that we must receive one reply for each message sent (half-duplex protocol)" but the docs don't say that.

(In fact the strange looking perl example filter looks broken in that regard. I guess it predates the decision to use strict half-duplex.)

They also don't say that the response must be JSON otherwise you get an "invalid reply" error. That sure seems like a bug to me (shouldn't it only be calling MsgSetPropsViaJSON if INPUT_JSON?) or at least a major doc omission. Or, better still, provide an interface.output config option (I've opened a case for that).