nixos/udev: update description for services.udev.path

services.udev.path already contains some defaults, these are however
defined in the module implementation, not options.

Update the description to make this more clear.

nixos/modules/services/hardware/udev.nix

@@ -222,6 +222,9 @@ in
         description = lib.mdDoc ''
           Packages added to the {env}`PATH` environment variable when
           executing programs from Udev rules.
+
+          coreutils, gnu{sed,grep}, util-linux and config.systemd.package are
+          automatically included.
         '';
       };