[css-color-4] clarify no pre-iterpolation gamut mapping, fix w3c#5928

Author: svgeesus

css-color-4/Overview.bs

@@ -4409,7 +4409,12 @@ Color space for interpolation</h3>
 	This also means that authors can choose to opt-in to Lab interpolation
 	by using the non-legacy ''color(srgb r g b)'' form.
 
-	Issue(5928):
+	If the colors to be interpolated are outside the gamut
+	of the interpolation space,
+	then once converted to that space,
+	they will contain out of range values.
+
+	These are not clipped, but the values are interpolated as-is.