Routing: Tidy and remove unknown component processing rule

[hamishwillee]

This tidies up the routing rules in the MAVLink guide to improve English, and remove out of date and/or incorrect instructions.

1. id -> ID
2. Components execute code, not systems. Where context is handling/routing, system -> component.
3. Instructions stated how autopilot and GCS system ids allocated in overview. Moved that to an id allocation section and noted it is a convention.
4. Instructions had that old info about MAV_COMPONENT being used for component IDs. Made clear you can use it, but not assume component type.

The main impeteus was to remove the processing criteria:

• The target_system matches its system ID and the component is unknown (i.e. this component has not seen any messages on any link that have the message's target_system/target_component).

This states that a component should attempt to process messages intended for its system if it has never seen the component before. For example, an autopilot sees a CAMERA message targeted at its system with component ID that it has not seen before it should attempt to handle the command.
That doesn't make sense: if the message has a target component id then it should be handled by that component.
In this case, the routing rules would attempt to get the message to its target.

ArduPilot does not do this: ArduPilot/ardupilot#33493

PX4 also doesn't implement it. The matching logic only matches on broadcast and specific component id.

[hamishwillee]

This is the clause that is dropped - the others say the same thing, but the wording is simplified.

I did think this might be a miss-applied routing rule. I don't think it adds value though, and it contradicts the idea that you only route if you have previously seen something from the component.

[hamishwillee]

@peterbarker @julianoes Here is the update to routing rules. Sorry, more than just removing the offending line (though I have done that). I confirmed PX4 definitely does not do this, and I still think it makes no sense.

[peterbarker]

@Hwurzburg we have much the same text on the Wiki: https://ardupilot.org/dev/docs/mavlink-routing-in-ardupilot.html#detailed-theory-of-mavlink-routing