tablet-v2: clarify whether zwp_tablet_v2.id is required
Right now there is no hint saying that zwp_tablet_v2.id
is required or optional.
The sentence "This event is sent in the initial burst of events before the wp_tablet.done event" is also used in the optional zwp_tablet_v2.path
event right below, so doesn't mean that it's required.
It's useful to make the event optional for virtual devices.
cc @whot