Input: document the MT event slot protocol

This patch adds documentation for the ABS_MT_SLOT event and gives
examples of how to use the event slot protocol.
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>