Also document around/exception behavior.

If this were a unit test, this would be a strange place to be talking about behavior which is implemented down in example.rb.

But from the point of view of documentation, I think it could be quite helpful to tell people that they don't need to mess around with `ensure` to make sure their cleanup runs when writing an around hook. This came up in code review for me today.

I'd title this differently, `Scenario: An around hook continues to run even if the example throw an exception` because the utility of around hooks isn't limited to cleanup.