[css-position] Elaborate a little bit on the issue about sticky and margin collapsing.

Author: dbaron

css-position/Overview.bs

@@ -581,9 +581,12 @@ Sticky positioning</h3>
   <p class="issue">
     Say what happens if it already overflows the containing block
   </p>
-  <p class="issue">
+  <p class="issue" id="issue-sticky-margin-collapsing">
     Do margins collapse between the stickily positioned element and its containing
     block element?
+    More generally, what margin is this referring to?
+    Does it involve collapsing between the sticky element and
+    its descendants, siblings, and/or ancestors?
   </p>
 
   Intersection between the stickily positioned element and the bottom of the