[Improvement] Area: add change event (#1019)

docs/markdown/en-US/area.md

@@ -55,6 +55,7 @@ Set `columns-num` with 2, you'll have a 2 level picker.
 |-----------|-----------|-----------|
 | confirm | triggers when clicking the confirm button | an array |
 | cancel | triggers when clicking the cancel button | - |
+| change | Triggered when current option changed | Picker instance, current values，column index |
 
 ### Data Structure
 

docs/markdown/en-US/picker.md

@@ -145,7 +145,7 @@ Picker events will pass different parameters according to the columns are single
 |-----------|-----------|-----------|
 | confirm | Triggered when click confirm button | Single column：current value，current index<br>Multiple columns：current values，current indexes |
 | cancel | Triggered when click cancel button | Single column：current value，current index<br>Multiple columns：current values，current indexes |
-| change | Triggered when current option changed | Single column：current value，current index<br>Multiple columns：current values，column index |
+| change | Triggered when current option changed | Single column：Picker instance, current value，current index<br>Multiple columns：Picker instance, current values，column index |
 
 
 ### Data struct of columns

docs/markdown/zh-CN/area.md

@@ -53,6 +53,7 @@ Vue.use(Area);
 | --- | --- | --- |
 | confirm | 点击右上方完成按钮 | 一个数组参数，具体格式看下方数据格式章节 |
 | cancel | 点击取消按钮时 | - |
+| change | 选项改变时触发 | Picker 实例，所有列选中值，当前列对应的索引 |
 
 ### 数据格式
 

docs/markdown/zh-CN/picker.md

@@ -147,7 +147,7 @@ Picker 组件的事件会根据 columns 是单列或多列返回不同的参数
 |-----------|-----------|-----------|
 | confirm | 点击完成按钮时触发 | 单列：选中值，选中值对应的索引<br>多列：所有列选中值，所有列选中值对应的索引 |
 | cancel | 点击取消按钮时触发 | 单列：选中值，选中值对应的索引<br>多列：所有列选中值，所有列选中值对应的索引 |
-| change | 选项改变时触发 | 单列：选中值，选中值对应的索引<br>多列：所有列选中值，当前列对应的索引 |
+| change | 选项改变时触发 | 单列：Picker 实例，选中值，选中值对应的索引<br>多列：Picker 实例，所有列选中值，当前列对应的索引 |
 
 
 ### Columns 数据结构