Camera protocol - update with info about camera addressing and missions

[hamishwillee]

Work related to https://github.com/mavlink/mavlink/pull/2024

This documents the presence of the id parameter in the image command. I've tried to make it clear how addressing works both before and after, and in the command protocol and missions.

@julianoes @rmackay9 @Davidsastresas A couple of things popped up:

1. If you're doing the interactive case you need to first query if the camera is ready and what it's properties are. It may be that we should add docs to indicate that MAV_CMD_REQUEST_MESSAGE for getting the camera status can use the id as param2, allowing you to just query status on a particular camera.
2. How can a system tell if the id is not supported? My guess is that there isn't any way, since I doubt either PX4 or ArduPilot mandated param1 be zero. Does it matter?