No polling_interval doc if no polling operation

https://github.com/Azure/azure-sdk-for-python/pull/12768/files#diff-640194745c3a66bc02afb2eef3f4321cR26

![image](https://user-images.githubusercontent.com/1050156/88724233-fdf17b80-d0de-11ea-89b0-99fa53d08738.png)

If there is no polling operation in entire SDK, we shall not see that doc