[css-box-4] Add issue

Author: fantasai

css-box-4/Overview.bs

@@ -430,6 +430,8 @@ Trimming Block Container Content</h4>
 		<li>Any margin collapsed with these margins.
 	</ul>
 
+	ISSUE(6922): Define effect on inlines (if any) and in inline axis (if any).
+
 	It also affects floats
 	for which the [=block container=] is a [=containing block=] by:
 	<ul>