In [bug report] version 2.11.1, the date picker value change event written on the document is change, but the actual measurement is input event. Please pay attention to updating the document. Thank you.

### Element UI version
2.11.1

### OS/Browsers version
Chrome 79.0.3945.88

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/bruski_wang/pen/XWJRwZO

### Steps to reproduce
1. 使用 选择日期后无反应
2. 使用 选择日期后打印变化的值

### What is Expected?
date-picker使用文档上说的change事件获取日期输入的变化值

### What is actually happening?
date-picker使用文档上 说的change事件无效， input事件才有效，估计是文档忘记更新。