[css-color-4] clarify image-p3 transfer function, fix w3c#295

Author: svgeesus

css-color-4/Overview.bs

@@ -1922,8 +1922,10 @@ Predefined colorspaces: ''srgb'', ''image-p3'', and ''rec2020''.</h3>
 			The ''srgb'' [[!SRGB]] colorspace accepts three numeric parameters,
 			representing the red, green, and blue channels of the color,
 			with each having a valid range of [0, 1].
+			The whitepoint is D65
+			(a daylight white, with a correlated color temperature of 6504°K).
 
-			It is the default colorspace for CSS,
+			sRGB is the default colorspace for CSS,
 			identical to specifying a color with the ''rgb()'' function.
 
 			It has the following characteristics:
@@ -1948,9 +1950,11 @@ Predefined colorspaces: ''srgb'', ''image-p3'', and ''rec2020''.</h3>
 
 		<dt><dfn>image-p3</dfn>
 		<dd>
-			The ''image-p3'' [[!DCI-P3]] colorspace accepts three numeric parameters,
+			The ''image-p3'' colorspace accepts three numeric parameters,
 			representing the red, green, and blue channels of the color,
 			with each having a valid range of [0, 1].
+			It uses the same primary chromaticities as [[!DCI-P3]],
+			but with a D65 whitepoint and the same transfer curve as sRGB.
 
 			It has the following characteristics: