(#18895) Fix documentation in runit provider

The runit provider had a couple of statements in documentation and error
messages that were inconsistent with what happens in the code 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`.