sane service: mention the lp group for printer+scanners

nixos/modules/services/hardware/sane.nix

···
         Enable support for SANE scanners.
 
         <note><para>
-          Users in the "scanner" group will gain access to the scanner.
+          Users in the "scanner" group will gain access to the scanner, or the "lp" group if it's also a printer.
         </para></note>
       '';
     };