media: cec-ioc-g-mode.rst: improve description of message, processing

The description of how messages are processed by the core was not
always very clear. Reword it to improve this.

In particular for the USER_CONTROL_* messages a critical bit was
missing in that the core also checks for the CEC_LOG_ADDRS_FL_ALLOW_RC_PASSTHRU
flag. This was confusing.

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