lib: clarify warning about mdDoc

lib/options.nix

···
      Kept here to alert downstream users who may not be aware of the migration's
      completion that it should be removed from modules.
   */
-  mdDoc = lib.warn "lib.mdDoc was removed from nixpkgs. Option descriptions are now in Markdown by default, you can remove any remaining uses of it.";
+  mdDoc = lib.warn "lib.mdDoc will be removed from nixpkgs in 24.11. Option descriptions are now in Markdown by default; you can remove any remaining uses of lib.mdDoc.";
 
   /* For use in the `defaultText` and `example` option attributes. Causes the
      given MD text to be inserted verbatim in the documentation, for when