[media] pulse8-cec: add notes about behavior in autonomous mode

The pulse8 dongle has some quirky behaviors when in autonomous mode.
Document these.

Signed-off-by: Johan Fjeldtvedt <jaffe1@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>