Third threading rule could be more clear about use of JoinAsync #697

Related to:
https://github.com/microsoft/vs-threading/blob/master/doc/threading_rules.md#rule-3-if-ever-awaiting-work-that-was-started-earlier-that-work-must-be-joined

The documentation for Rule #3 is not clear regarding the use of JoinAsync in asynchronous methods. There is brief mention of await longRunningWork, but there is no explanation that this is equivalent to using JoinAsync(CancellationToken.None). I would suggest documenting it as JoinAsync(cancellationToken), followed by a note saying that a shorter form is available if the cancellation token is not required.