I think the ``versionadded`` directive must be present here. Otherwise, after 2.5 has been released, you'll automatically get this version of the docs by default which could then be confusing if you are using an older Symfony version and don't notice that you can't use the ``getEvent()`` method then.

Thus, I'd add this above the new paragraph:
```rst
.. versionadded:: 2.5
    The ``getEvent()`` method was introduced in Symfony 2.5.
```