message: clarify a comment
diff: @@ -386,6 +386,7 @@ impl<'mir, 'tcx: 'mir> ThreadManager<'mir, 'tcx> {
  data_race: Option<&mut data_race::GlobalState>,
  ) -> InterpResult<'tcx> {
  if self.threads[joined_thread_id].join_status == ThreadJoinStatus::Detached {
  + // On Windows this corresponds to joining on a closed handle.
  throw_ub_format!("trying to join a detached thread");
  }
  