Added more dispatcher descriptions

akka-docs/java/dispatchers.rst

@@ -42,6 +42,8 @@ There are 4 different types of message dispatchers:
 
 * Dispatcher
 
+  - This is an event-based dispatcher that binds a set of Actors to a thread pool. It is the default dispatcher used if one is not specified.
+
   - Sharability: Unlimited
 
   - Mailboxes: Any, creates one per Actor
@@ -55,6 +57,8 @@ There are 4 different types of message dispatchers:
 
 * PinnedDispatcher
 
+  - This dispatcher dedicates a unique thread for each actor using it; i.e. each actor will have its own thread pool with only one thread in the pool.
+
   - Sharability: None
 
   - Mailboxes: Any, creates one per Actor
@@ -66,6 +70,10 @@ There are 4 different types of message dispatchers:
 
 * BalancingDispatcher
 
+  - This is an executor based event driven dispatcher that will try to redistribute work from busy actors to idle actors. 
+
+  - It is assumed that all actors using the same instance of this dispatcher can process all messages that have been sent to one of the actors; i.e. the actors belong to a pool of actors, and to the client there is no guarantee about which actor instance actually processes a given message.
+
   - Sharability: Actors of the same type only
 
   - Mailboxes: Any, creates one for all Actors
@@ -79,6 +87,10 @@ There are 4 different types of message dispatchers:
 
 * CallingThreadDispatcher
 
+  - This dispatcher runs invocations on the current thread only. This dispatcher does not create any new threads, 
+    but it can be used from different threads concurrently for the same actor. See :ref:`TestCallingThreadDispatcherRef`
+    for details and restrictions.
+
   - Sharability: Unlimited
 
   - Mailboxes: Any, creates one per Actor per Thread (on demand)