nixos/ddclient: add warning about password being world readable

Closes #16885.

(cherry picked from commit 78eac466b0f9ea964d7f8b3e473b93584c76bf44)

nixos/modules/services/networking/ddclient.nix

···
         default = "";
         type = str;
         description = ''
-          Password.
+          Password. WARNING: The password becomes world readable in the Nix store.
         '';
       };