Fix #16364 - Doc - Persistent actor - clarification of the recovery cust...

Persistent actor / clarification of the recovery customization 


C&P, [EventsourcedProcessor](http://doc.akka.io/api/akka/2.3.7/?_ga=1.187818158.760573030.1401365146#akka.persistence.EventsourcedProcessor)
 does not say that this is  **user-overridable** method so I'm not sure if this spec should contains it at all ?

```
  override def preRestart(reason: Throwable, message: Option[Any]) {
    processorStash.unstashAll()
    super.preRestart(reason, message)
  }
```