[Bug] Docs of the GROUP share level is incorrect.

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/kyuubi/issues?q=is%3Aissue) and found no similar issues.

### Describe the bug

The [doc](https://kyuubi.readthedocs.io/en/master/deployment/engine_share_level.html#group) of GROUP share level says
```
If the primary group is not found, it falls back to the USER level.
```
This is incorrect because
1. The default HadoopGroupProvider returns the username when the group list is empty, but the share level doesn't fallback to `USER`
2. We can't make this assumption to all `GroupProvider`s as users can implement their own versions.

### Affects Version(s)

master

### Kyuubi Server Log Output

_No response_

### Kyuubi Engine Log Output

_No response_

### Kyuubi Server Configurations

_No response_

### Kyuubi Engine Configurations

_No response_

### Additional context

_No response_

### Are you willing to submit PR?

- [ ] Yes. I would be willing to submit a PR with guidance from the Kyuubi community to fix.
- [X] No. I cannot submit a PR at this time.

Open deployment/engine_share_level.html and read the GROUP share-level section. Verify the documented fallback claim against the behavior described in the issue, then revise the section so it does not assert an unsupported USER fallback; the documentation should accurately describe GROUP behavior for different GroupProvider implementations.

Written by the indexing model from the issue text.