[Doc] add build-in style document (#633)

packages/switch-cell/index.vue

@@ -1,5 +1,5 @@
 <template>
-  <cell :title="title" class="van-switch-cell">
+  <cell :title="title" :border="border" class="van-switch-cell">
     <van-switch v-bind="$props" @input="$emit('input', $event)" />
   </cell>
 </template>
@@ -20,6 +20,7 @@ export default create({
   props: {
     title: String,
     value: Boolean,
+    border: Boolean,
     loading: Boolean,
     disabled: Boolean
   },

packages/vant-css/src/common/hairline.css

@@ -1,20 +1,14 @@
 @import "../mixins/border-retina.css";
 
-.van-hairline {
-  &,
-  &--top,
-  &--left,
-  &--right,
-  &--bottom,
-  &--top-bottom,
-  &--surround {
-    position: relative;
+[class*='van-hairline'] {
+  position: relative;
 
-    &::after {
-      @mixin hairline;
-    }
+  &::after {
+    @mixin hairline;
   }
+}
 
+.van-hairline {
   &--top::after {
     border-top-width: 1px;
   }