!act,doc #3831 Adding more Java with Lambda documentation and support

* The Java with Lambda support documentation for AbstractActor and AbstractFSM are now on par with Scala
* Many small fixes and additions of missing things
* Added an AbstractActorContext that has convenience functions for getChild and getChildren
Merge pull request #2051 from akka/wip-3831-java-8-doc-ban

!act,doc #3831 Adding more Java with Lambda documentation and support