[mediaqueries-4] Update color-gamut value text

Make it clear that it is the combination of the
User Agent and output device that needs to support
the colorspace.

Closes #4281.

Author: grorg

mediaqueries-4/Overview.bs

@@ -1630,21 +1630,21 @@ Color Display Quality: the 'color-gamut' feature</h3>
 	<dl dfn-type=value dfn-for="@media/color-gamut">
 		<dt><dfn>srgb</dfn>
 		<dd>
-			The output device can support approximately the sRGB gamut or more.
+			The UA and output device can support approximately the sRGB gamut or more.
 
 			Note: It is expected that the vast majority of color displays
 			will be able to return true to a query of this type.
 
 		<dt><dfn>p3</dfn>
 		<dd>
-			The output device can support approximately the gamut
+			The UA and output device can support approximately the gamut
 			specified by the DCI P3 Color Space or more.
 
 			Note: The ''p3'' gamut is larger than and includes the ''srgb'' gamut.
 
 		<dt><dfn>rec2020</dfn>
 		<dd>
-			The output device can support approximately the gamut
+			The UA and output device can support approximately the gamut
 			specified by the ITU-R Recommendation BT.2020 Color Space or more.
 
 			Note: The ''rec2020'' gamut is larger than and includes the ''p3'' gamut.