Operator doc for flatmap merge (#28488)

akka-docs/src/test/java/jdocs/stream/operators/sourceorflow/FlatMapMerge.java

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+/*
+ * Copyright (C) 2019-2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.sourceorflow;
+
+import akka.NotUsed;
+import akka.actor.ActorSystem;
+import akka.stream.javadsl.Source;
+
+import java.util.Arrays;
+
+public class FlatMapMerge {
+  private static ActorSystem system = null;
+
+  // #flatmap-merge
+  // e.g. could be a query to a database
+  private Source<String, NotUsed> lookupCustomerEvents(String customerId) {
+    return Source.from(Arrays.asList(customerId + "-evt-1", customerId + "-evt-2"));
+  }
+  // #flatmap-merge
+
+  void example() {
+    // #flatmap-merge
+    Source.from(Arrays.asList("customer-1", "customer-2"))
+        .flatMapMerge(10, this::lookupCustomerEvents)
+        .runForeach(System.out::println, system);
+    // prints - events from different customers could interleave
+    // customer-1-evt-1
+    // customer-2-evt-1
+    // customer-1-evt-2
+    // customer-2-evt-2
+    // #flatmap-merge
+  }
+}

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

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+/*
+ * Copyright (C) 2019-2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sourceorflow
+
+import akka.NotUsed
+import akka.actor.ActorSystem
+import akka.stream.scaladsl.Source
+
+object FlatMapMerge {
+
+  implicit val system: ActorSystem = ActorSystem()
+
+  // #flatmap-merge
+  val source: Source[String, NotUsed] = Source(List("customer-1", "customer-2"))
+
+  // e.g. could b a query to a database
+  def lookupCustomerEvents(customerId: String): Source[String, NotUsed] = {
+    Source(List(s"$customerId-evt-1", s"$customerId-evt2"))
+  }
+
+  source.flatMapMerge(10, customerId => lookupCustomerEvents(customerId)).runForeach(println)
+
+  // prints - events from different customers could interleave
+  // customer-1-evt-1
+  // customer-2-evt-1
+  // customer-1-evt-2
+  // customer-2-evt-2
+  // #flatmap-merge
+
+}