Update recommended way to shut down detached Yarn session cluster

When starting a Yarn session cluster in detached mode, then it is printed that one should shut down the cluster via yarn application -kill. The problem of this approach is, however, that temporary files won't be cleaned up which remain on HDFS. Hence, I think a better way would be to reconnect via the yarn-session.sh tool and then to stop the cluster gracefully. This won't leave orphaned files behind.