doc: clarify behaviour of writeFile(fd)

This is a continuing source of confusion, attempt to make it even more
clear.

Fixes: https://github.com/nodejs/node/issues/24923

PR-URL: https://github.com/nodejs/node/pull/27282


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>