fix #13337 by mentioning removal of .progress-* contextual classes

docs/migration.html

@@ -410,6 +410,11 @@ lead: "Guidance on how to upgrade from Bootstrap v2.x to v3.x with emphasis on m
           <td><code>.help-inline</code></td>
           <td>No exact equivalent, but <code>.help-block</code> is similar.</td>
         </tr>
+        <tr>
+          <td>Non-bar-level progress colors</td>
+          <td><code>.progress-info</code> <code>.progress-success</code> <code>.progress-warning</code> <code>.progress-danger</code></td>
+          <td>Use <code>.progress-bar-*</code> on the <code>.progress-bar</code> instead.</td>
+        </tr>
       </tbody>
     </table>
   </div><!-- /.table-responsive -->