- Clarify that if any service is specified it's used _instead of_, not
in addition to, the default hypothes.is service

- Add note that services settings are still experimental

- Edit the text for clarity

I still don't think the `authority` setting is very well described here,
but I think that setting may be going away in the future, so I'm not
going to try and document it better right now.

The `grantToken` setting could still do with much better docs as well.