Input: multitouch - augment event semantics documentation

Through the collaboration to adapt the N-trig and Stantum HID
drivers to the MT protocol, some semantic clarifications to the
protocol have been made. This patch adds them to the MT documentation.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Tested-by: Stéphane Chatty <chatty@enac.fr>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>