[css-color-5] Clarified that component keywords can return ''none'' as well as a number, fix #10151

Author: svgeesus

css-color-5/Overview.bs

@@ -918,7 +918,7 @@ but they all follow a common structure:
 	and attempting to do so is an error.
 * However, the [=origin color=] can use either modern or legacy syntax.
 
-The [=component keywords=] return a <<number>>;
+The [=component keywords=] return a <<number>>, or ''none'';
 if they were originally specified as a <<percentage>> or an <<angle>>,
 that <<percentage>> is resolved to a <<number>>
 and the <<angle>> is resolved to a <<number>> of degrees