Mention backdrop option in docs

docs/javascript.html

@@ -186,6 +186,12 @@ $('.navmenu').offcanvas()
           <td>true</td>
           <td>Disable scrolling when the off canvas element is shown, by setting overflow to hidden for the body.</td>
         </tr>
+        <tr>
+          <td>backdrop</td>
+          <td>boolean</td>
+          <td>false</td>
+          <td>If backdrop should be shown when menu is opened, in modal-like style.</td>
+        </tr>
       </tbody>
     </table>