Add corners stub making it clear we have no resolution yet on this

Author: LeaVerou

css-borders-4/Overview.bs

@@ -827,6 +827,11 @@ To interpolate a [=superellipse parameter=] |s| to an interpolation value betwee
 	</dl>
 </div>
 
+<h3 id=corners-shorthand>
+Corner Shaping Shorthand: the 'corners' property</h3>
+
+Issue(11623): Decide on the exact name and syntax of this property.
+
 <h2 id="partial-borders">
 Partial borders</h2>