[KYUUBI #3621][DOCS] Add a hint in create batch REST api

### _Why are the changes needed?_

Add a hint in creating batch REST API.

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [x] [Run test](https://kyuubi.apache.org/docs/latest/develop_tools/testing.html#running-tests) locally before make a pull request

Closes #3622 from deadwind4/create-batch-rest-doc.

Closes #3621

0d1e676 [Luning Wang] [KYUUBI #3621][DOCS] Add a hint in create batch REST api

Authored-by: Luning Wang <[email protected]>
Signed-off-by: Cheng Pan <[email protected]>

docs/client/rest/rest_api.md

@@ -289,6 +289,8 @@ Returns all the batches.
 
 ### POST /batches
 
+Create a new batch.
+
 #### Request Body
 
 | Name      | Description                                        | Type             |