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.

Good suggestion, I've made that change. In addition to the reason you give, it is more clear (the wording "Clean up" could mean a lot of things, but "around hook continues to run" is more specific).