docs: Document Table column[key] can be ignored.

Related #6436

components/table/index.en-US.md

@@ -83,7 +83,7 @@ One of Property `columns` for descriping column, Column has the same API.
 | Property      | Description              | Type            |  Default     |
 |---------------|--------------------------|-----------------|--------------|
 | title      | title of this column        | string\|ReactNode | - |
-| key        | key of this column | string          | - |
+| key        | key of this column, you can ignore thie prop if you've set a unique `dataIndex` | string          | - |
 | dataIndex  | display field of the data record, could be set like `a.b.c` | string | - |
 | render     | renderer of table cell, has three params: text, record and index of this row. The render value should be a ReactNode, or a object for [colSpan/rowSpan config](#demo-colspan-rowspan) | Function(text, record, index) {} | - |
 | filters    | filter menu config        | object[]       | - |

components/table/index.zh-CN.md

@@ -84,7 +84,7 @@ const columns = [{
 | 参数       | 说明                       | 类型            |  默认值  |
 |-----------|----------------------------|-----------------|---------|
 | title      | 列头显示文字               | string\|ReactNode | - |
-| key        | React 需要的 key，建议设置 | string          | - |
+| key        | React 需要的 key，如果已经设置了唯一的 `dataIndex`，可以忽略这个属性 | string          | - |
 | dataIndex  | 列数据在数据项中对应的 key，支持 `a.b.c` 的嵌套写法 | string | - |
 | render     | 生成复杂数据的渲染函数，参数分别为当前行的值，当前行数据，行索引，@return里面可以设置表格[行/列合并](#demo-colspan-rowspan) | Function(text, record, index) {} | - |
 | filters    | 表头的筛选菜单项           | object[]           | - |