#1697: update upgrading docs to include mention of no more secondary class in modals

docs/templates/pages/upgrading.mustache

@@ -76,6 +76,7 @@
     <li>{{_i}}IE9: removed gradients and added rounded corners{{/i}}</li>
     <li>{{_i}}Updated active state to make styling clearer in button groups (new) and look better with custom transition{{/i}}</li>
     <li>{{_i}}New mixin, <code>.buttonBackground</code>, to set button gradients{{/i}}</li>
+    <li>{{_i}}The <code>.secondary</code> class was removed from modal examples in our docs as it never had associated styles.{{/i}}</li>
   </ul>
   <h3>{{_i}}Forms{{/i}}</h3>
   <ul>