InputEvent.device: it's not documented what value corresponds to "All devices".

**Your Godot version:**
Any.

**Issue description:**
It seems that value of [`InputMap.ALL_DEVICES`](https://github.com/godotengine/godot/blob/b9a2787bd164602e866ecac1527020e0e1c34da8/core/input/input_map.cpp#L39) constant is not documented anywhere.
I suggest exposing that constant in the documentation (edit: and to scripting) and adding some reference to it in the description of `InputEvent.device`.

**URL to the documentation page (if already existing):**
- `InputMap`: [3.2](https://docs.godotengine.org/en/3.2/classes/class_inputmap.html), [stable](https://docs.godotengine.org/en/stable/classes/class_inputmap.html), [latest](https://docs.godotengine.org/en/latest/classes/class_inputmap.html),
- `InputEvent.device`: [3.2](https://docs.godotengine.org/en/3.2/classes/class_inputevent.html#class-inputevent-property-device), [stable](https://docs.godotengine.org/en/stable/classes/class_inputevent.html#class-inputevent-property-device), [latest](https://docs.godotengine.org/en/latest/classes/class_inputevent.html#class-inputevent-property-device).