Add documentation around InputEvent Device ID

[donovani]

Your Godot version:
4.1.1 C# Windows

Issue description:
InputEvent.Device does not have much documentation around it, & gives users the impression that it is a unique ID per device. However, (at least on my editor) it appears that the behavior is instead to assign device IDs that are unique to the device type (keyboard, mouse, joypad). If this is behavior that is consistent across platforms then this would be extremely helpful to document, given that this number is used by multiple methods of Input.

Valdiation:
Given the following code:

public partial class Foobar : Node3D
{
	public override void _Input(InputEvent e)
	{
		GD.Print(e.Device + "\t" + e.AsText());
	}
}

1. Make an input with the keyboard - device ID is 0
2. Make an input with the mouse - device ID is 0
3. Make an input with a joypad - device ID is 0
4. Make an input with a second joypad - device ID is 1

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/classes/class_inputevent.html#property-descriptions

[Marenz]

Related: godotengine/godot-proposals#7161

[Marenz]

And also related: If you want to add an input mapping valid for all devices from code using InputMap.action_add_event, you must use -1 for the object passed to the event parameter.
This is also not documented.

[ievr]

Got bitten by this not being documented and against initial intuition. One has to have two controllers to discover that the identifier counts per-device-type.

The relevant file is this one.
doc/classes/InputEvent.xml