Configuration/Cluster Set Up chapter miss information about ERL_EPMD_ADDRESS env variable

When setting-up couchdb cluster following documentation at https://docs.couchdb.org/en/stable/setup/cluster.html there is no indication for the need to set the ERL_EPMD_ADDRESS environment variable to expose epmd to other nodes, in particular when using the couchdb provided debian package where this variable is defined in `/etc/default/couchdb` to `127.0.0.1`

I think this should be added to the documentation to avoid loosing time when configuring a cluster.

Read the cluster setup chapter at docs.couchdb.org/en/stable/setup/cluster.html and compare its guidance with the Debian package setting in /etc/default/couchdb. Update the documentation to explain when ERL_EPMD_ADDRESS must expose epmd beyond localhost, and verify that the cluster setup steps cover this configuration.

Written by the indexing model from the issue text.