fs: document why isPerformingIO is required

PR-URL: https://github.com/nodejs/node/pull/33982


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>