=doc #3886 improved example of PartialFunction.orElse composition

Changed `PartialFunction#orElse` composition example to better highlight it's possibilities. 

This is a follow up to a discussion on akka-user: https://groups.google.com/forum/#!topic/akka-user/vVUZPMeJmzw

Opened issue for this here: https://www.assembla.com/spaces/akka/simple_planner#/ticket:3886

Existing docs: http://doc.akka.io/docs/akka/snapshot/scala/actors.html#Extending_Actors_using_PartialFunction_chaining