Add documentation for axis updates without a tracking ID

If a touch starts and terminates while in SYN_DROPPED, the tracking ID appears
to stay at -1, but the other axes may update. We need to pass these on to the
client since the kernel may buffer the next event with the same value
otherwise. Note this in the documentation so that client's don't create touch
points based on out-of-touchpoint updates.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>