media: cec-core.rst: improve cec_transmit_done documentation

Clarify that calling cec_transmit_done can start a new transmit and
that you should put the hardware in a state that allows for a new
transmit before calling this function.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>