media: cec-core.rst/cec-ioc-receive.rst: clarify CEC_TX_STATUS_ERROR

CEC_TX_STATUS_ERROR can be used if the HW cannot tell LOST_ARB and
LOW_DRIVE apart, or when some other error occurs. It is not a
replacement for NACK.

So the hardware must be able to tell the difference between OK, NACK and
'something else'.

Clarify the documentation (both public and kernel API) on this point.

Also fix two small typos (this messages -> this message).

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