media: media/doc: improve bt.2020 documentation

Add a note stating that bt.2020 is often used in combination with the
smpte 2084 transfer function.

Also use the right references to the documentation of that transfer
function.
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/colorspaces-defs.rst

@@ -97,7 +97,7 @@ whole range, 0-255, dividing the angular value by 1.41. The enum
     * - ``V4L2_XFER_FUNC_DCI_P3``
       - Use the DCI-P3 transfer function.
     * - ``V4L2_XFER_FUNC_SMPTE2084``
-      - Use the SMPTE 2084 transfer function.
+      - Use the SMPTE 2084 transfer function. See :ref:`xf-smpte-2084`.
 
 
 

Documentation/media/uapi/v4l/colorspaces-details.rst

@@ -418,6 +418,11 @@ Inverse Transfer function:
 
     L = \left( \frac{L' + 0.099}{1.099}\right) ^{\frac{1}{0.45} }\text{, for } L' \ge 0.081
 
+Please note that while Rec. 709 is defined as the default transfer function
+by the :ref:`itu2020` standard, in practice this colorspace is often used
+with the :ref:`xf-smpte-2084`. In particular Ultra HD Blu-ray discs use
+this combination.
+
 The luminance (Y') and color difference (Cb and Cr) are obtained with
 the following ``V4L2_YCBCR_ENC_BT2020`` encoding: