document async success callback in changelog

index.html

@@ -4231,6 +4231,10 @@ ActiveRecord::Base.include_root_in_json = false
         Added an overridable <tt>_isModel</tt> for more advanced
         control of what's considered a model by your Collection.
       </li>
+      <li>
+        The <tt>success</tt> callback passed to <tt>Model#destroy</tt> is always
+        called asynchronously now.
+      </li>
       <li>
         <tt>Router#execute</tt> passes back the route name as its third argument.
       </li>