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: #31365

PR-URL: #31367
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/stream.md

@@ -2639,12 +2639,22 @@ const finished = util.promisify(stream.finished);
 
 const writable = fs.createWriteStream('./file');
 
+function drain(writable) {
+  if (writable.destroyed) {
+    return Promise.reject(new Error('premature close'));
+  }
+  return Promise.race([
+    once(writable, 'drain'),
+    once(writable, 'close')
+      .then(() => Promise.reject(new Error('premature close')))
+  ]);
+}
+
 async function pump(iterable, writable) {
   for await (const chunk of iterable) {
     // Handle backpressure on write().
     if (!writable.write(chunk)) {
-      if (writable.destroyed) return;
-      await once(writable, 'drain');
+      await drain(writable);
     }
   }
   writable.end();