Cluster Formation guide: rework automatic node cleanup section
Cluster Formation guide: rework automatic node cleanup section

To document `cluster_formation.node_cleanup.interval` and
explain what the negative consequences are.

References rabbitmq/discussions#113

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website