doc: add a note to `buf.fill()` description

PR-URL: https://github.com/nodejs/node/pull/25547


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>