media: buffer.rst: clean up timecode documentation

V4L2_BUF_FLAG_TIMECODE is not video capture specific, so drop that
part.

The 'Timecodes' section was a bit messy, so that's cleaned up.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>