Input: alps - document stick behavior for protocol V2

Document that protocol V2 uses standard (bare) PS/2 mouse packets for the
DualPoint stick.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-By: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>