[scroll-animations-1] Address remarks

Author: bramus

scroll-animations-1/Overview.bs

@@ -184,7 +184,10 @@ spec: cssom-view-1; type: dfn;
 
 		<dt><dfn>self</dfn>
 		<dd>
-			Specifies to use the element’s own box as the [=scroll container=].
+			Specifies to use the element’s own [=principal box=] as the
+			[=scroll container=]. If the [=principal box=] is not a 
+			[=scroll container=], or if there is no [=scrollable overflow=], 
+			then the [=scroll progress timeline=] is [=inactive timeline|inactive=].
 	</dl>
 
 ### The {{ScrollTimeline}} Interface ### {#scrolltimeline-interface}