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>

Documentation/media/kapi/cec-core.rst

@@ -268,6 +268,10 @@ to 1, if the hardware does support retry then either set these counters to
 0 if the hardware provides no feedback of which errors occurred and how many
 times, or fill in the correct values as reported by the hardware.
 
+Be aware that calling these functions can immediately start a new transmit
+if there is one pending in the queue. So make sure that the hardware is in
+a state where new transmits can be started *before* calling these functions.
+
 The cec_transmit_attempt_done() function is a helper for cases where the
 hardware never retries, so the transmit is always for just a single
 attempt. It will call cec_transmit_done() in turn, filling in 1 for the