clarify what rec.2020 is actually used for

Author: svgeesus

css-color-4/Overview.bs

@@ -2068,7 +2068,7 @@ Predefined colorspaces: ''srgb'', ''image-p3'', ''a98rgb'', ''prophotorgb'' and
 		<dd>
 			The ''rec2020'' [[!Rec.2020]] colorspace accepts three numeric parameters,
 			representing the red, green, and blue channels of the color,
-			with each having a valid range of [0, 1].
+			with each having a valid range of [0, 1]. ITU Reference 2020 is used for High Definition, 4k and 8k television.
 
 			It has the following characteristics: