nixos/kanata: add default value, doc and warning for devices

nixos/modules/services/hardware/kanata.nix

···
     options = {
       devices = mkOption {
         type = types.listOf types.str;
+        default = [ ];
         example = [ "/dev/input/by-id/usb-0000_0000-event-kbd" ];
-        description = mdDoc "Paths to keyboard devices.";
+        description = mdDoc ''
+          Paths to keyboard devices.
+
+          An empty list, the default value, lets kanata detect which
+          input devices are keyboards and intercept them all.
+        '';
       };
       config = mkOption {
         type = types.lines;
···
   };
 
   config = mkIf cfg.enable {
+    warnings =
+      let
+        keyboardsWithEmptyDevices = filterAttrs (name: keyboard: keyboard.devices == [ ]) cfg.keyboards;
+        existEmptyDevices = length (attrNames keyboardsWithEmptyDevices) > 0;
+        moreThanOneKeyboard = length (attrNames cfg.keyboards) > 1;
+      in
+      optional (existEmptyDevices && moreThanOneKeyboard) "One device can only be intercepted by one kanata instance.  Setting services.kanata.keyboards.${head (attrNames keyboardsWithEmptyDevices)}.devices = [ ] and using more than one services.kanata.keyboards may cause a race condition.";
+
     hardware.uinput.enable = true;
 
     systemd.services = mapAttrs' mkService cfg.keyboards;