bug#38929: sane-hpaio not found by simple-scan?

[raid5atemyhomework]

I have a USB-connected HP DeskJet Ink Advantage 1515 printer/scanner.

On my own system (this is a full Guix System, not Guix on a foreign distro) I 
modified the configuration.scm:

```scheme
(use-package-modules #;... scanner)

(operating-system
  #;...
  (services
    (append
      #;...
      (modify-services %desktop-services
        (sane-service-type _ => sane-backends))))
  #;...)
```

After `sudo guix system reconfigure configuration.scm`, `simple-scan` still 
could not find it.  However, on reboot, it was able to find my 
already-connected USB printer/scanner without any issues.

Perhaps only some kind of force-trigger of `udev` would be needed after the 
above setup, but `udev` is pretty early in the Shepherd tree.

On the other hand the `sane-service-type` documentation in `info guix` is 
***severely*** lacking, and the documentation is somehow lost in the middle of 
documentation about geolocation services, so it should probably be fixed.

Thanks
raid5atemyhomework