[css-color-4] Clarify legacy syntax does not allow none. w3c#7816

Author: svgeesus

css-color-4/Overview.bs

@@ -956,7 +956,9 @@ any achromatic color will have a [=powerless=] hue component.
 
 	It can also be specified explicitly,
 	by providing the keyword <dfn value for="<color>">none</dfn> for a component in a color function.
-	All color functions allow any of their components to be specified as ''none''.
+	All color functions 
+	(with the exception of those using the <a>legacy color syntax</a>) 
+	allow any of their components to be specified as ''none''.
 
 	For handling of [=missing component=] in color interpolation,
 	see [[#interpolation-missing]]