=doc #3539 Update the cluster spec

* The cluster spec has been rearranged into two sections describing the current cluster and future enhancements/additions.
* The state diagram has been replaced with a new one describing the current cluster implementation.
* A number of small additons and changes to the text.
Merge pull request #1689 from akka/wip-3539-clearly-separate-and-update-current-and-future-cluster-doc-2.2-ban

=doc #3539 Update the cluster spec (backport)