[KYUUBI #4119][FOLLOWUP] Add app start time for batch api docs

### _Why are the changes needed?_

#4119 follow up

### _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

- [ ] [Run test](https://kyuubi.readthedocs.io/en/master/develop_tools/testing.html#running-tests) locally before make a pull request

Closes #4365 from turboFei/app_start.

Closes #4119

3cf7e7a [fwang12] add app start time

Authored-by: fwang12 <fwang12@ebay.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>
(cherry picked from commit e647cc9)
Signed-off-by: Cheng Pan <chengpan@apache.org>

docs/client/rest/rest_api.md

@@ -493,6 +493,7 @@ The [Engine](#engine) List.
 | user           | The user created the batch                                        | String |
 | batchType      | The batch type                                                    | String |
 | name           | The batch name                                                    | String |
+| appStartTime   | The batch application start time                                  | Long   |
 | appId          | The batch application Id                                          | String |
 | appUrl         | The batch application tracking url                                | String |
 | appState       | The batch application state                                       | String |