DOC: Add docs about Stash

Two problems:

API docs

The ScalaDoc does not even compile. I assume the code should be:

class ActorWithProtocol extends Actor with Stash {
     def receive = {
       case "open" ⇒
         unstashAll() 
         context.become {
           case "write" ⇒ // do writing...
           case "close" ⇒
             unstashAll()
             context.unbecome()
           case msg ⇒ stash()
         }
       case "done" ⇒ // done
       case msg    ⇒ stash()
     }
   }

Missing Documentation

Not clear if it works for Java API. If so, document both. If not, document that as well.

Current ScalaDoc:

The Stash trait enables an actor to temporarily stash away messages that can not or should not be handled using the actor's current behavior.

Example:

   class ActorWithProtocol extends Actor with Stash {
     def receive = {
       case "open" ⇒
         unstashAll {
           case "write" ⇒ // do writing...
           case "close" ⇒
             unstashAll()
             context.unbecome()
           case msg ⇒ stash()
         }
       case "done" ⇒ // done
       case msg    ⇒ stash()
     }
   }
Note that the Stash trait can only be used together with actors that have a deque-based mailbox. Actors can be configured to use a deque-based mailbox using a configuration like the following (see the documentation on dispatchers on how to configure a custom dispatcher):

 akka {
   actor {
     my-custom-dispatcher {
       mailbox-type = "akka.dispatch.UnboundedDequeBasedMailbox"
     }
   }
 }
Note that the Stash trait must be mixed into (a subclass of) the Actor trait before any trait/class that overrides the preRestart callback. This means it's not possible to write Actor with MyActor with Stash if MyActor overrides preRestart.