message: Explain why we don't duplicate panics from other threads
diff: @@ -354,7 +354,12 @@ where
  },
  });
  - let result = future.wait().unwrap_or_else(|| Canceled::throw());
  + let result = future.wait().unwrap_or_else(|| {
  + // If the other thread panics, we treat this as cancelation: there is no
  + // need to panic ourselves, since the original panic will already invoke
  + // the panic hook and bubble up to the thread boundary (or be caught).
  + Canceled::throw()
  + });
  ProbeState::UpToDate(if result.cycle.is_empty() {
  Ok(result.value)
  } else {
  