=doc #15565 improved wording in cluster-usage docs

Resolves #15565

Needs backport to 2.3
Yes, true - that sentence is somehow hard to split into `or` with examples though, so how about splitting it up like this:

```
If you don't configure seed nodes you need to join the cluster programmatically or manually.

Manual joining can be performed by using ref:`cluster_jmx_java` or :ref:`cluster_command_line_java`.
Joining programatically can be performed with ``Cluster.get(system).join``. 

You can join to any node in the cluster. It does not have to be configured as a seed node.
Note that you can only join to an existing cluster member, which means that for bootstrapping some
node must join itself.
```

And pulled out distinct "how to join" and "to whom can I join" sections. Better / worse?