docs: update doc for "Import Code Snippets" (close: #581)

docs/guide/markdown.md

@@ -272,9 +272,19 @@ module.exports = {
 
 You can import code snippets from existing files via following syntax:
 
-**Input**
+``` md
+<<< @/filepath
+```
+
+It also supports [line highlighting](#line-highlighting-in-code-blocks):
 
+``` md
+<<< @/filepath{highlightLines} 
 ```
+
+**Input**
+
+``` md
 <<< @/test/markdown/fragments/snippet.js{2}
 ```
 

docs/zh/guide/markdown.md

@@ -270,6 +270,16 @@ module.exports = {
 
 你可以通过下述的语法导入已经存在的文件中的代码段：
 
+``` md
+<<< @/filepath
+```
+
+它也支持 [行高亮](#代码块中的行高亮)：
+
+``` md
+<<< @/filepath{highlightLines} 
+```
+
 **Input**
 
 ```