doc: update write callback documentation

- replace _*may or may not* be called_ with _will be called_ because the
  callback is always called
- remove _To reliably detect write errors, add a listener for the
  `'error'` event_ because the `error` event will NOT be emitted if a
  write occurs after the stream has been closed

PR-URL: https://github.com/nodejs/node/pull/38959
Fixes: https://github.com/nodejs/node/issues/38704


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>