doc: hide undocumented object artifacts in async_hooks

The examples show `process.stdout.fd` as a means to use synchronous
writes in async_hooks context. However this is an undocumented field,
so showcase a file write example instead.

Fixes: https://github.com/nodejs/node/issues/22873

PR-URL: https://github.com/nodejs/node/pull/24741


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>