Document scanAsync (#28239)

* Document scanAsync

* Include async function in scanAsync example

akka-docs/src/test/scala/docs/stream/operators/sourceorflow/ScanAsync.scala

@@ -3,26 +3,34 @@
  */
 
 package docs.stream.operators.sourceorflow
+
 import akka.stream.scaladsl.Source
 
+import scala.concurrent.ExecutionContext
 import scala.concurrent.Future
 
 object ScanAsync {
+
   def scanAsyncExample(): Unit = {
     import akka.actor.ActorSystem
 
     implicit val system: ActorSystem = ActorSystem()
+    implicit val ec: ExecutionContext = system.dispatcher
+
+    //#scan-async
+    def asyncFunction(acc: Int, next: Int): Future[Int] = Future {
+      acc + next
+    }
 
-    //#scanAsync
     val source = Source(1 to 5)
-    source.scanAsync(0)((acc, x) => Future.successful(acc + x)).runForeach(println)
+    source.scanAsync(0)((acc, x) => asyncFunction(acc, x)).runForeach(println)
     // 0  (= 0)
     // 1  (= 0 + 1)
     // 3  (= 0 + 1 + 2)
     // 6  (= 0 + 1 + 2 + 3)
     // 10 (= 0 + 1 + 2 + 3 + 4)
     // 15 (= 0 + 1 + 2 + 3 + 4 + 5)
-    //#scanAsync
+    //#scan-async
   }
 
 }