Serialization Docs: add info about adding a logger (#22618)

akka-docs/rst/java/code/jdocs/serialization/SerializationDocTest.java

@@ -21,6 +21,11 @@ public class SerializationDocTest {
   //#my-own-serializer
   public class MyOwnSerializer extends JSerializer {
 
+    // If you need logging here, introduce a constructor that takes an ExtendedActorSystem.
+    // public MyOwnSerializer(ExtendedActorSystem actorSystem) 
+    // Get a logger using:
+    // private final LoggingAdapter logger = Logging.getLogger(actorSystem, this); 
+
     // This is whether "fromBinary" requires a "clazz" or not
     @Override public boolean includeManifest() {
       return false;

akka-docs/rst/scala/code/docs/serialization/SerializationDocSpec.scala

@@ -20,6 +20,11 @@ package docs.serialization {
   //#my-own-serializer
   class MyOwnSerializer extends Serializer {
 
+    // If you need logging here, introduce a constructor that takes an ExtendedActorSystem.
+    // class MyOwnSerializer(actorSystem: ExtendedActorSystem) extends Serializer
+    // Get a logger using:
+    // private val logger = Logging(actorSystem, this)
+
     // This is whether "fromBinary" requires a "clazz" or not
     def includeManifest: Boolean = true