doc: update table ajax demo #4980

close #4980

Author: tangjinzhou

components/table/demo/ajax.vue

@@ -12,10 +12,14 @@ title:
 
 另外，本例也展示了筛选排序功能如何交给服务端实现，列不需要指定具体的 `onFilter` 和 `sorter` 函数，而是在把筛选和排序的参数发到服务端来处理。
 
+当使用 `rowSelection` 时，请设置 `rowSelection.preserveSelectedRowKeys` 属性以保留 `key`。
+
 ## en-US
 
 This example shows how to fetch and present data from a remote server, and how to implement filtering and sorting in server side by sending related parameters to server.
 
+Setting `rowSelection.preserveSelectedRowKeys` to keep the `key` when enable selection.
+
 **Note, this example use [Mock API](https://randomuser.me) that you can look up in Network Console.**
 </docs>