[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>

Documentation/media/uapi/v4l/control.rst

@@ -137,6 +137,12 @@ Control IDs
 ``V4L2_CID_GAIN`` ``(integer)``
     Gain control.
 
+    Primarily used to control gain on e.g. TV tuners but also on
+    webcams. Most devices control only digital gain with this control
+    but on some this could include analogue gain as well. Devices that
+    recognise the difference between digital and analogue gain use
+    controls ``V4L2_CID_DIGITAL_GAIN`` and ``V4L2_CID_ANALOGUE_GAIN``.
+
 ``V4L2_CID_HFLIP`` ``(boolean)``
     Mirror the picture horizontally.