Document DNS names in cluster file

FDB 7.1 supports to use DNS names in a cluster file to connect to a FDB cluster. We should document the requirements and how to make use of this new feature. The FDB Kubernetes operator already has support for this new feature: https://github.com/FoundationDB/fdb-kubernetes-operator/issues/968. For reference we have the same issue in the operator repository, having the general setup documented would help writing the docs for the operator: https://github.com/FoundationDB/fdb-kubernetes-operator/issues/1419.

Once the setup is documented we can close: https://github.com/apple/foundationdb/issues/354