message: Tag: update document
diff: <template>
  - <span class="van-tag" :class="[type ? `van-tag--${type}`: '', { 'is-plain': plain, 'is-mark': mark }]">
  + <span :class="['van-tag', { [`van-tag--${type}`]: type, 'is-plain': plain, 'is-mark': mark }]">
  <slot></slot>
  </span>
  </template>
  