[css-values-5] Clarify parse-time validity of arbitrary substitution functions

Closes #13083

Author: nex3

css-values-5/Overview.bs

@@ -3938,7 +3938,8 @@ Resolving in Properties</h3>
 	</div>
 
 	If a property value contains one or more [=arbitrary substitution functions=],
-	and those functions are themselves syntactically valid,
+	and all of those functions are themselves syntactically valid
+	according to their [=argument grammar=]s,
 	the entire value's grammar must be assumed to be valid at parse time.
 
 	<wpt pathprefix="css/css-variables">