[css-color-4] Clarify calc and none, fix #6920

Author: svgeesus

css-color-4/Overview.bs

@@ -609,10 +609,14 @@ any achromatic color will have a [=powerless=] hue component.
 	in the appropriate unit for that component: ''0'', ''0%'', or ''0deg''.
 	This includes rendering the color directly,
 	converting it to another color space,
+	performing computations on the color component values,
 	etc.
 
 	If a color with a [=missing component=] is serialized
 	or otherwise presented directly to an author,
+	then for <a>legacy color syntax</a>
+	it represents that component as a zero value;
+	otherwise,
 	it represents that component as being the ''none'' keyword.
 
 	<div class=example id="ex-missing-hue" class="non-normative">