modules/nixpkgs: warn about changing defaults

If end-users change the nixpkgs revision to something other than our
default, they can run into issues not picked up by our test suite.

wrappers/modules/nixpkgs.nix

@@ -40,6 +40,9 @@ in
       description = ''
         Whether Nixvim should use the ${config.meta.wrapper.name} configuration's `pkgs`,
         instead of constructing its own instance.
+
+        > [!CAUTION]
+        > Changing this option can lead to issues that may be difficult to debug.
       '';
     };
   };