Documentation bug: interface

The documentation at https://jobqueue.dask.org/en/latest/generated/dask_jobqueue.SLURMCluster.html (and the one for the other schedulers) says

> interfacestr
> Network interface like ‘eth0’ or ‘ib0’. This will be used both for the Dask scheduler and the Dask workers interface. If you need a different interface for the Dask scheduler you can pass it through the scheduler_options argument: interface=your_worker_interface, scheduler_options={'interface': your_scheduler_interface}.

It's unclear what happens if one doesn't specify it. Looking at the code it appears that a default is used, which is taken from the config. The default config.yaml file has `null` value for interface so even looking in there one goes around in circle.

I propose:

1. adding the sentence "If no `interfacestr` is specified the default from the `jobqueue.yaml` file is utilized" to the documentation
2. adding yet another sentence saying what happens if no specific `jobqueue.yaml` file is present (so the one from the install directory must be used).

What happens in the second scenario is still unclear to me. On the machine I use it clearly works, so some interface is utilized, but which one? I have both eth0 and ib0. Is it guaranteed to work always if at least one interface is present? How is it chosen if more than one is present?