doc: clarify that `v8.serialize()` is not deterministic

Fixes: https://github.com/nodejs/help/issues/2448

PR-URL: https://github.com/nodejs/node/pull/31548


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>