+clu #3636 Revert join to uninitialized

* Revert the change introduced in
  https://github.com/akka/akka/pull/1738/files
* The cleanup/improvements aside of the actual
  feature is not reverted by this patch
* Clarify the documentation
Merge pull request #1765 from akka/wip-3636-revert-join-uninitialized-patriknw

+clu #3636 Revert join to uninitialized