DOC: Integrating with a new event loop in the kernel

["Integrating with a new event loop in the kernel"](https://ipython.readthedocs.io/en/stable/config/eventloops.html?#integrating-with-a-new-event-loop-in-the-kernel) section has not been updated for version 5.

Since gh-323, do_one_iteration method has become a coroutine and Tk integration is not using it anymore. Please update the documentation to explain how do_one_iteration can be used.

See https://github.com/KxSystems/pyq/issues/99 for an example of a custom kernel that no longer works properly.