DOC: Integrating with a new event loop in the kernel #381

"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 KxSystems/pyq#99 for an example of a custom kernel that no longer works properly.