Add scanAsync examples to akka-docs (#25468) (#27990)

* Add scanAsync examples to akka-docs (#25468)

* Sync scan with scanAsync in akka-docs (#25468)

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

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sourceorflow
+import akka.stream.scaladsl.Source
+
+import scala.concurrent.Future
+
+object ScanAsync {
+  def scanAsyncExample(): Unit = {
+    import akka.actor.ActorSystem
+
+    implicit val system: ActorSystem = ActorSystem()
+
+    //#scanAsync
+    val source = Source(1 to 5)
+    source.scanAsync(0)((acc, x) => Future.successful(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
+  }
+
+}