Improve the documentation for "What restarting means"

Akka docs, 2.3.3:

suspend the actor
call the old instance’s supervisionStrategy.handleSupervisorFailing method (defaults to suspending all children)
call the old instance’s preRestart hook (defaults to sending termination requests to all children and calling postStop)
wait for all children stopped during preRestart to actually terminate
call the old instance’s supervisionStrategy.handleSupervisorRestarted method (defaults to sending restart request to all remaining children)
create new actor instance by invoking the originally provided factory again
invoke postRestart on the new instance
resume the actor

Improvements:

Clarify what suspend and resume mean in detail, e.g. no longer process user messages
Clarify how waiting for children works
supervisionStrategy.handleSupervisorRestarted removed in 2.1 (accoring to @rkuhn)
Sending restart requests to remaining children after creating the new instance (and postRestart, resume ?) (according to @rkuhn)