add documentation for the TraceableEventDispatcher class
feature #3916 [Component][EventDispatcher] documentation for the TraceableEventDispatcher (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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1160f7c add documentation for the TraceableEventDispatcher class
Merge branch '2.3' into 2.4