Adding example of cancelled operator (#29749)

Co-authored-by: Arnout Engelen <github@bzzt.net>
Co-authored-by: Renato Cavalcanti <renato@cavalcanti.be>

akka-docs/src/main/paradox/stream/operators/Sink/cancelled.md

@@ -13,6 +13,16 @@ Immediately cancel the stream
 
 Immediately cancel the stream
 
+## Example
+
+In this example, we have a source that generates numbers from 1 to 5 but as we have used cancelled we get `NotUsed` as materialized value and stream cancels.
+
+Scala
+:   @@snip [Cancelled.scala](/akka-docs/src/test/scala/docs/stream/operators/sink/Cancelled.scala) { #cancelled }
+
+Java
+:   @@snip [SinkDocExamples.java](/akka-docs/src/test/java/jdocs/stream/operators/SinkDocExamples.java) { #cancelled }
+
 ## Reactive Streams semantics
 
 @@@div { .callout }
@@ -20,5 +30,3 @@ Immediately cancel the stream
 **cancels** immediately
 
 @@@
-
-

akka-docs/src/test/java/jdocs/stream/operators/SinkDocExamples.java

@@ -114,6 +114,14 @@ public class SinkDocExamples {
     // #fold
   }
 
+  static NotUsed cancelledExample() {
+    // #cancelled
+    Source<Integer, NotUsed> source = Source.range(1, 5);
+    NotUsed sum = source.runWith(Sink.cancelled(), system);
+    return sum;
+    // #cancelled
+  }
+
   static void headOptionExample() {
     // #headoption
     Source<Integer, NotUsed> source = Source.empty();

akka-docs/src/test/scala/docs/stream/operators/sink/Cancelled.scala

@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sink
+
+import akka.NotUsed
+import akka.actor.ActorSystem
+import akka.stream.scaladsl.{ Sink, Source }
+
+import scala.concurrent.ExecutionContextExecutor
+
+object Cancelled {
+  implicit val system: ActorSystem = ???
+  implicit val ec: ExecutionContextExecutor = system.dispatcher
+  def cancelledExample(): NotUsed = {
+    //#cancelled
+    val source = Source(1 to 5)
+    source.runWith(Sink.cancelled)
+    //#cancelled
+  }
+}