Added doc about message ordering guarantees.

Signed-off-by: Jonas Bonér <jonas@jonasboner.com>

akka-docs/scala/actors.rst

@@ -34,7 +34,7 @@ Here is an example:
 
   import akka.actor.Actor
   import akka.event.EventHandler
-  
+
   class MyActor extends Actor {
     def receive = {
       case "test" => EventHandler.info(this, "received test")
@@ -160,7 +160,7 @@ mailbox and the hotswap stack are unaffected by the restart, so processing of
 messages will resume after the :meth:`postRestart` hook returns. Any message
 sent to an actor while it is being restarted will be queued to its mailbox as
 usual.
- 
+
 Stop Hook
 ^^^^^^^^^
 
@@ -218,6 +218,8 @@ Messages are sent to an Actor through one of the following methods.
     additionally the old method could defer possible type cast problems into
     seemingly unrelated parts of the code base.
 
+Message ordering is guaranteed on a per-sender basis.
+
 Fire-forget
 ^^^^^^^^^^^
 
@@ -639,16 +641,16 @@ In generic base Actor:
 .. code-block:: scala
 
   import akka.actor.Actor.Receive
-  
+
   abstract class GenericActor extends Actor {
     // to be defined in subclassing actor
     def specificMessageHandler: Receive
-   
+
     // generic message handler
     def genericMessageHandler: Receive = {
       case event => printf("generic: %s\n", event)
     }
-   
+
     def receive = specificMessageHandler orElse genericMessageHandler
   }
 
@@ -661,5 +663,5 @@ In subclassing Actor:
       case event: MyMsg  => printf("specific: %s\n", event.subject)
     }
   }
-  
+
   case class MyMsg(subject: String)