minor #3931 [Component][EventDispatcher] 2.5 specific documentation for the TraceableEventDispatcher (xabbuh)

This PR was merged into the 2.5 branch.

Discussion
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[Component][EventDispatcher] 2.5 specific documentation for the TraceableEventDispatcher

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.5+
| Fixed tickets | #3879

Commits
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cf667c9 update the TraceableEventDispatcher to reflect its movement to the EventDispatcher component in Symfony 2.5

Author: weaverryan

components/event_dispatcher/traceable_dispatcher.rst

@@ -5,13 +5,17 @@
 The Traceable Event Dispatcher
 ==============================
 
-The :class:`Symfony\\Component\\HttpKernel\\Debug\\TraceableEventDispatcher`
+.. versionadded:: 2.5
+    The ``TraceableEventDispatcher`` class was moved to the EventDispatcher
+    component in Symfony 2.5. Before, it was located in the HttpKernel component.
+
+The :class:`Symfony\\Component\\EventDispatcher\\Debug\\TraceableEventDispatcher`
 is an event dispatcher that wraps any other event dispatcher and can then
 be used to determine which event listeners have been called by the dispatcher.
 Pass the event dispatcher to be wrapped and an instance of the
 :class:`Symfony\\Component\\Stopwatch\\Stopwatch` to its constructor::
 
-    use Symfony\Component\HttpKernel\Debug\TraceableEventDispatcher;
+    use Symfony\Component\EventDispatcher\Debug\TraceableEventDispatcher;
     use Symfony\Component\Stopwatch\Stopwatch;
 
     // the event dispatcher to debug