message: docs: add docs for builtin classes
diff: @@ -89,3 +89,21 @@ Enable safe area.
  <div v-show="visible">Slide Right</div>
  </transition>
  ```
  +
  +### Haptics Feedback
  +
  +Add haptics feedback for an element. When touched, the opacity of the element is reduced.
  +
  +Usually used in clickable elements such as button.
  +
  +```html
  +<div class="van-haptics-feedback"></div>
  +```
  +
  +### Clearfix
  +
  +Clear floated content within a container.
  +
  +```html
  +<div class="van-clearfix"></div>
  +```
  