[css-grid-3][masonry] Mark issue wrt formatting contexts.

Author: fantasai

css-grid-3/Overview.bs

@@ -365,7 +365,7 @@ Establishing Masonry Layout</h3>
 	</dl>
 
 	A [=masonry container=] that is not [=subgridded=] in its [=grid axis=] establishes
-	an [=independent formatting context|independent=] <dfn export>grid formatting context</dfn>
+	an [=independent formatting context=]
 	for its contents.
 
 	<div class=issue>
@@ -374,6 +374,10 @@ Establishing Masonry Layout</h3>
 	Suggestions are welcome in the issue.
 	</div>
 
+	<div class=issue>
+	ISSUE(12820): Write up how grid/masonry formatting contexts work more formally?
+	</div>
+
 <h2 id="masonry-track-templates">
 Masonry Track Specification</h2>