Update cluster-usage.rst

Fixing https://github.com/akka/akka/issues/15565 by making it clear when programmatic joining is required.
What about:

If you do not have configured seed nodes, then you need to join manually or programmaticaly, using :ref:`cluster_jmx_scala`, :ref:`cluster_command_line_scala` or the API. You can join to any node in the cluster. It doesn't have to be configured as a seed node.

Joining can be performed programatically with ``Cluster(system).join``...