doc: improve worker pool example

In the worker pool example, the 'kWorkerFreedEvent' should be emitted
in case of error as well. After adding new worker in the error handler,
the pending tasks should be notified of an available worker.

PR-URL: #33082
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Juan José Arboleda <[email protected]>

doc/api/async_hooks.md

@@ -820,6 +820,7 @@ class WorkerPool extends EventEmitter {
     });
     this.workers.push(worker);
     this.freeWorkers.push(worker);
+    this.emit(kWorkerFreedEvent);
   }
 
   runTask(task, callback) {