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>