[css-color-4][editorial] Added clarifying note to alpha value syntax, that modern formats also take the value none.

This was clear in the individual syntax definitions but could mislead someone reading this section by itself.

Author: svgeesus

css-color-4/Overview.bs

@@ -1099,6 +1099,12 @@ Representing Transparency in Colors: the <<alpha-value>> syntax</h3>
 	<dfn dfn-for="<color>">&lt;alpha-value></dfn> = <<number>> | <<percentage>>
 	</pre>
 
+	Note: This syntax is used as-is for [=legacy color syntax=].
+	For [=modern color syntax=],
+	it is extended
+	so that alpha components
+	can also take the value ''none''.
+
 	Unless otherwise specified,
 	an <<alpha-value>> component of a color defaults to ''100%'' when omitted.
 	Values outside the range [0,1] are not invalid,