media: cec: document the new CEC pin capability, events and mode

Document CEC_CAP_MONITOR_PIN, CEC_EVENT_PIN_LOW/HIGH,
CEC_EVENT_FL_DROPPED_EVENTS and CEC_MODE_MONITOR_PIN.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>