media: vidioc-g-dv-timings.rst: document V4L2_DV_FL_CAN_DETECT_REDUCED_FPS

Document the new V4L2_DV_FL_CAN_DETECT_REDUCED_FPS flag and
update the V4L2_DV_FL_REDUCED_FPS description since it can now
also be used with receivers.

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