(#18895) Fix documentation in runit provider

The documentation and error messages for the runit provider made a couple of
statements that were inconsistent with the code:

  - `/etc` is not checked as a daemon directory. The path checked is `/etc/sv`.

  - `/var/lib/service` is checked as a daemon directory, but this wasn't
    documented.

  - The service directory `/etc/service` is checked before `/var/service`.