Clarify that USE_LONGNAME can be required for consul and other clustering setups

[lukebakken]

trafficstars

https://github.com/rabbitmq/rabbitmq-server/discussions/4229

The documentation here implies that cluster_formation.consul.use_longname = true is all you need to do to use long node names:

https://www.rabbitmq.com/cluster-formation.html#peer-discovery-consul

It should have the same note about USE_LONGNAME as in other sections of the page.