[css-fonts-4] Clarified that override-colors are invalid if an absolute color is not specified, w3c#9555

Author: svgeesus

css-fonts-4/Overview.bs

@@ -7862,7 +7862,8 @@ Overriding a colors from a palette: The 'override-colors!!descriptor' descriptor
 	to the initial color palette represented by this ''@font-palette-values'' rule.
 
 	The specified <<color>> must be an
-	[=absolute color=].
+	[=absolute color=],
+	otherwise it is invalid.
 
 	The '@font-palette-values/override-colors' descriptor takes
 	a comma-separated list of palette index entries and colors.