doc: improve fs.read documentation

PR-URL: https://github.com/nodejs/node/pull/29270


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>