docs(extensions): improved cases for using @/...

Author: wenfangdu

docs/rules/extensions.md

@@ -109,6 +109,8 @@ import foo from './foo';
 import bar from './bar';
 
 import Component from './Component';
+
+import foo from '@/foo';
 ```
 
 The following patterns are not considered problems when configuration set to "always":
@@ -121,6 +123,8 @@ import bar from './bar.json';
 import Component from './Component.jsx';
 
 import * as path from 'path';
+
+import foo from '@/foo.js';
 ```
 
 The following patterns are considered problems when configuration set to "ignorePackages":
@@ -145,6 +149,7 @@ import Component from './Component.jsx';
 
 import express from 'express';
 
+import foo from '@/foo'
 ```
 
 The following patterns are not considered problems when configuration set to `['error', 'always', {ignorePackages: true} ]`:
@@ -156,6 +161,7 @@ import baz from 'foo/baz.js';
 
 import express from 'express';
 
+import foo from '@/foo';
 ```
 
 ## When Not To Use It