Example for Source.fromIterator #25468 (#28582)

* add example for Source.fromIterator #25468

* scala fmt code style fix

* don't use test as a sample for docs

akka-docs/src/test/java/jdocs/stream/operators/source/From.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.stream.operators.source;
+
+import akka.actor.ActorSystem;
+import akka.stream.javadsl.Source;
+
+import java.util.Arrays;
+
+public class From {
+
+  private ActorSystem system = null;
+
+  void fromIteratorSample() {
+    // #from-iterator
+    Source.fromIterator(() -> Arrays.asList(1, 2, 3).iterator())
+        .runForeach(System.out::println, system);
+    // could print
+    // 1
+    // 2
+    // 3
+    // #from-iterator
+  }
+}