media: cec-ioc-receive.rst: clarify sequence and status fields

Improve the documentation for CEC_RECEIVE/TRANSMIT w.r.t. the
sequence and tx/rx_status fields.

Also remove a duplicate tx_status description.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>