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