Better documentation around cancellation tokens

Our C# docs need expanded content around timeouts and cancellation. The guidance should cover:

- an outline of the relevant scenarios (the host/app is restarting, an execution has hit the execution time limit, an event hubs partition lease was lost)
- demonstrate how to modify a function to accept a `CancellationToken`
- demonstrate checking for cancellation manually, and passing the token through to async APIs
- explain why following this guidance is important (give examples of what happens if you don't do this)

One other issue we need to figure out is how to approach this same topic for other languages because the story is not as good there (we don't have an equivalent of CancellationToken in JavaScript)