Update DatePicker docs (#15092)

components/date-picker/index.en-US.md

@@ -77,7 +77,7 @@ The following APIs are shared by DatePicker, MonthPicker, RangePicker, WeekPicke
 
 | Property | Description | Type | Default |
 | -------- | ----------- | ---- | ------- |
-| defaultValue | to set default date | [moment](http://momentjs.com/) | - |
+| defaultValue | to set default date, if start time or end time is null or undefined, the date range will be an open interval | [moment](http://momentjs.com/) | - |
 | defaultPickerValue | to set default picker date | [moment](http://momentjs.com/) | - |
 | disabledTime | to specify the time that cannot be selected | function(date) | - |
 | format | to set the date format, refer to [moment.js](http://momentjs.com/). When an array is provided, all values are used for parsing and first value is used for formatting. | string \| string[] | "YYYY-MM-DD" |

components/date-picker/index.zh-CN.md

@@ -78,7 +78,7 @@ moment.locale('zh-cn');
 
 | 参数 | 说明 | 类型 | 默认值 |
 | --- | --- | --- | --- |
-| defaultValue | 默认日期 | [moment](http://momentjs.com/) | 无 |
+| defaultValue | 默认日期，如果开始时间或结束时间为 `null` 或者 `undefined`，日期范围将是一个开区间 | [moment](http://momentjs.com/) | 无 |
 | defaultPickerValue | 默认面板日期 | [moment](http://momentjs.com/) | 无 |
 | disabledTime | 不可选择的时间 | function(date) | 无 |
 | format | 设置日期格式，为数组时支持多格式匹配，展示以第一个为准。配置参考 [moment.js](http://momentjs.com/) | string \| string[] | "YYYY-MM-DD" |