[media] v4l: controls: Improve documentation for V4L2_CID_GAIN

Elaborate the differences between V4L2_CID_GAIN and gain-type specific
V4L2_CID_DIGITAL_GAIN and V4L2_CID_ANALOGUE_GAIN.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>