nixos/systemd-tmpfiles: use types.attrsWith for settings

That way it's way easier to derive from the manual which key is used for
what exactly.

nixos/modules/system/boot/systemd/tmpfiles.nix

@@ -7,6 +7,10 @@ let
   initrdCfg = config.boot.initrd.systemd.tmpfiles;
   systemd = config.systemd.package;
 
+  attrsWith' = placeholder: elemType: types.attrsWith {
+    inherit elemType placeholder;
+  };
+
   settingsOption = {
     description = ''
       Declare systemd-tmpfiles rules to create, delete, and clean up volatile
@@ -25,7 +29,7 @@ let
       };
     };
     default = {};
-    type = types.attrsOf (types.attrsOf (types.attrsOf (types.submodule ({ name, config, ... }: {
+    type = attrsWith' "config-name" (attrsWith' "tmpfiles-type" (attrsWith' "path" (types.submodule ({ name, config, ... }: {
       options.type = mkOption {
         type = types.str;
         default = name;