Input: clarify the no-finger event in multitouch protocol

The current documentation does not explicitly specify how to report
zero fingers, leaving a potential problem in the driver implementations
and giving no parsing directive to userland. This patch defines two
equally valid ways.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>