[css-text-4] Link to issue

Author: fantasai

css-text-4/Overview.bs

@@ -10757,7 +10757,8 @@ Text Spacing Character Classes</h4>
 
 	In the context of this property the following definitions apply:
 
-	Issue: Classes and Unicode code points need to be reviewed.
+	Issue(9503): Classes and Unicode code points are under review,
+	and at least some changes are needed to accommodate more recent additions to Unicode.
 
 	<dl>
 		<dt><dfn>ideographs</dfn>