src: document DoWrite() usage expectations

Clarify how it must behave for both synchronous and asynchronous
completion.

PR-URL: https://github.com/nodejs/node/pull/26339


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>