[css-cascade-4] Add issue for defining whatever filtering requirements Shadow DOM needs.

Author: fantasai

css-cascade-4/Overview.bs

@@ -736,6 +736,8 @@ Filtering</h2>
 			and the declaration's value matches the syntax for that property.
 	</ul>
 
+	ISSUE: Define whatever it is that Shadow DOM needs here.
+
 	The values of the declarations that apply form,
 	for each property on each element,
 	a list of <a>declared values</a>.