ibus: add engine list to module description

nixos/modules/i18n/inputMethod/ibus.nix

@@ -28,10 +28,14 @@ in
         type    = with types; listOf ibusEngine;
         default = [];
         example = literalExample "with pkgs.ibus-engines; [ mozc hangul ]";
-        description = ''
-          Enabled IBus engines.
-          Available engines can be found by running `nix-env "<nixpkgs>" . -qaP -A ibus-engines`.
-        '';
+        description =
+          let
+            engines =
+              lib.concatStringsSep ", "
+              (map (name: "<literal>${name}</literal>")
+               (lib.attrNames pkgs.ibus-engines));
+          in
+            "Enabled IBus engines. Available engines are: ${engines}.";
       };
     };
   };