doc: add note about multiple sync events and once

Fixes: https://github.com/nodejs/node/issues/32431

PR-URL: https://github.com/nodejs/node/pull/34220


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>