=clu #3750 Update cluster specification

* Based on Jonas' notes: https://gist.github.com/jboner/7692270
* Also removed "Future Extension" descriptions of cluster actor reference,
  since that is probably not the direction we are taking
Merge pull request #1986 from akka/wip-3750-improve-cluster-specification-patriknw

=clu #3750 Update cluster specification