add 'viewport' option to popover docs; #12328 erratum

docs/_includes/js/popovers.html

@@ -220,6 +220,14 @@ $('.popover-dismiss').popover({
           <td>'click'</td>
           <td>How popover is triggered - click | hover | focus | manual</td>
         </tr>
+        <tr>
+         <td>viewport</td>
+         <td>string | object</td>
+         <td>{ selector: 'body', padding: 0 }</td>
+         <td>
+          <p>Keeps the popover within the bounds of this element. Example: <code>viewport: '#viewport'</code> or <code>{ selector: '#viewport', padding: 0 }</code></p>
+         </td>
+       </tr>
       </tbody>
     </table>
   </div><!-- /.table-responsive -->