I prefer to reorder this a bit, following the same structure as the HttpKernel component docs:

````rst
The ``PRE_SET_DATA`` event is dispatched in the beginning of the
``Form::setData()`` method. It can be used to:

* Modify the data given during pre-population;
* Modify a form depending on the pre-populated data (adding or removing fields dynamically).

The listeners recieve a :class:`Symfony\\Component\\Form\\FormEvent` instance
with the Model data, which can be modified if desired. The resulting data is
then used as the new model data.

.. sidebar:: ``PRE_SET_DATA`` in the Form component

    The :class:`Symfony\Component\Form\Extension\Core\EventListener\ResizeFormListener`
    subscriber subscribes to the ``PRE_SET_DATA`` event in order to reorder the
    form's fields depending on the data from the pre-populated object, by removing
    and adding all form rows.
````

Please note that I also removed some information. It isn't really the task of the Form Event documentation to tell what will happen with the model data passed to `setData`. That should be documented somewhere else, if it is usefull.