Surfacing submarines

(This might already be obvious to all involved, in which case let's make this a documentation issue.)

It is trivially easy to surface a submarine exception:

  def foo[F[_]](implicit F: Concurrent[F]): F[Unit] = {
    Handle.allowF[F, String] { handle =>
      F.start(handle.raise("foo"))
    }.rescue(_ => F.never).flatMap[Unit] { fib =>
      F.widen(fib.joinWithNever)
    }
  }

The idea is that we can use the Handle in a completely different fiber, while allowF/rescue is lexical.

Start by reading the Handle.allowF and rescue usage shown in the issue, along with the Concurrent fiber behavior involved in F.start and joinWithNever. Determine whether the behavior is already expected; if so, document how a Handle can be used across fibers and how lexical rescue applies, and make the documentation accurately reflect the example.