docs: improve code comments (#1911)

src/views/components-demo/back-to-top.vue

@@ -1,7 +1,7 @@
 <template>
   <div class="components-container">
-    <code>{{ $t('components.backToTopTips1') }}</code>
-    <code>{{ $t('components.backToTopTips2') }}</code>
+    <aside>{{ $t('components.backToTopTips1') }}</aside>
+    <aside>{{ $t('components.backToTopTips2') }}</aside>
     <div class="placeholder-container">
       <div>placeholder</div>
       <div>placeholder</div>
@@ -113,8 +113,7 @@
       <div>placeholder</div>
       <div>placeholder</div>
     </div>
-    <!--可自定义按钮的样式、show/hide临界点、返回的位置  -->
-    <!--如需文字提示，可在外部添加element的<el-tooltip></el-tooltip>元素  -->
+    <!-- you can add element-ui's tooltip -->
     <el-tooltip placement="top" content="tooltip">
       <back-to-top :custom-style="myBackToTopStyle" :visibility-height="300" :back-position="50" transition-name="fade" />
     </el-tooltip>
@@ -129,6 +128,7 @@ export default {
   components: { BackToTop },
   data() {
     return {
+      // customizable button style, show/hide critical point, return position
       myBackToTopStyle: {
         right: '50px',
         bottom: '50px',