[css-color-4] Make explicit that legacy forms do not support none. #7137

Author: svgeesus

css-color-4/Overview.bs

@@ -868,6 +868,7 @@ Legacy Comma-separated Color Function Syntax</h4>
 	* non-opaque forms use a separate notation
 		(for example ''hsla()'' rather than ''hsl()'')
 	* minimum required precision is lower, 8 bits per component
+	* the ''none'' value is not allowed
 
 	<div class=example id="example-rgba-legacy"p>
 		<p>The following represents a saturated sRGB red that is 50% opaque: