=doc #3886 improved example of PartialFunction.orElse composition

Changed orElse composition example to better highlight it's
possibilities. This is a follow up PR based on a discussion on
akka-user: https://groups.google.com/forum/#!topic/akka-user/vVUZPMeJmzw
Merge pull request #2023 from ktoso/doc-3886-partialfunc-orelse-ktoso

=doc #3886 improved example of PartialFunction.orElse composition