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