UnobservedTaskExceptionEventArgs.SetObserved mentions old default

For `UnobservedTaskExceptionEventArgs.SetObserved()` the remarks state:

> The Exception property is used to examine the exception without marking it as observed, whereas the method is used to mark the exception as observed. Marking the exception as observed prevents it from triggering exception escalation policy which, by default, terminates the process.

But "by default" is no longer true. The default does not terminate the process since .net 4.5 because `ThrowUnobservedTaskExceptions` is now false by default.