doc: further fix async iterator example

Further fixes an issue with the async iterator example where an
incorrect assumption was made in regards that drain or error
is always invoked after !write().

Fixes: https://github.com/nodejs/node/issues/31365

PR-URL: https://github.com/nodejs/node/pull/31367


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>