code: public final ParallelFlux<T> doOnCancel(Runnable onCancel) {
  		Objects.requireNonNull(onCancel, "onCancel");
  		return doOnSignal(this, null, null, null, null, null, null, null, onCancel);
  	}
docs: Run the specified runnable when a 'rail' receives a cancellation.
  
  @param onCancel the callback
  
  @return the new {@link ParallelFlux} instance