### _Why are the changes needed?_
- Update doc to mark the spark plugin's config `spark.sql.optimizer.insertRepartitionNum` used for Spark 3.1 only
### _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#4933 from bowenliang123/insert-num.
Closes#4933
5ed6e2867 [liangbowen] comment and style
280a6af03 [liangbowen] spark.sql.optimizer.insertRepartitionNum only available for Spark 3.1.x
7f01cf3b6 [liangbowen] spark.sql.optimizer.insertRepartitionNum only available for Spark 3.1.x
Authored-by: liangbowen <liangbowen@gf.com.cn>
Signed-off-by: liangbowen <liangbowen@gf.com.cn>
c8a138f986