doc: add DatePicker default-value

examples/docs/en-US/date-picker.md

@@ -262,6 +262,7 @@ Picking a date range is supported.
 | popper-class | custom class name for DatePicker's dropdown | string | — | — |
 | picker-options | additional options, check the table below | object | — | {} |
 | range-separator | range separator | string | - | ' - ' |
+| default-value | optional default time of the picker | Date | anything accepted by `new Date()` | - |
 
 ### Picker Options
 | Attribute      | Description          | Type      | Accepted Values       | Default  |

examples/docs/zh-CN/date-picker.md

@@ -296,6 +296,7 @@
 | popper-class | DatePicker 下拉框的类名 | string | — | — |
 |picker-options | 当前时间日期选择器特有的选项参考下表 | object |  — | {} |
 | range-separator | 选择范围时的分隔符 | string | - | ' - ' |
+| default-value | 可选，DatePicker打开时默认显示的时间 | Date | 可被new Date()解析 | - |
 
 ### Picker Options
 | 参数      | 说明          | 类型      | 可选值                           | 默认值  |