tablet-v2: clarify that name/id events are optional

libinput may not always have a descriptive name for a tablet
device, in which case it's better to let the Wayland client
pick a fallback (potentially localized) than send a fake string.

Not all tablet devices are USB, so make it clear that the id
event may be skipped.

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: wayland/wayland-protocols#180