fix(async): correct 'drain' method defs. Closes #41648 (#41665)

http://caolan.github.io/async/v3/docs.html#cargo
http://caolan.github.io/async/v3/docs.html#QueueObject

Thanks!

types/async/index.d.ts

@@ -106,7 +106,12 @@ export interface AsyncCargo {
     push(task: any, callback?: Function): void;
     saturated(): void;
     empty(): void;
-    drain(): void;
+    /**
+     * a function that sets a callback that is called when the last item from the queue has returned from the worker.
+     * If the callback is omitted, q.drain() returns a promise for the next occurrence.
+     */
+    drain(): Promise<void>;
+    drain(handler: () => void): void;
     idle(): boolean;
     pause(): void;
     resume(): void;

types/async/test/index.ts

@@ -256,6 +256,8 @@ const cargo = async.cargo((tasks, callback) => {
     }
     callback();
 }, 2);
+cargo.drain(); // $ExpectType Promise<void>
+cargo.drain(() => { console.log('done processing queue'); }); // $ExpectType void
 
 // add some items
 cargo.push({ name: 'foo' }, (err: Error) => { console.log('finished processing foo'); });