[Component][EventDispatcher] documentation for the TraceableEventDispatcher

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3, 2.4
| Fixed tickets | #3879

After this has been merged, the document has to be updated for Symfony 2.5 and higher to reflect that the ``TraceableEventDispatcher`` is now part of the EventDispatcher component.