Explained a more simple method of dynamic form handling available since #8827

https://github.com/symfony/symfony/pull/8827

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no https://github.com/symfony/symfony/pull/8827
| Applies to    | 2.2.6+
| Fixed tickets | -

This PR explains the changes in the above mentioned PR.

Also I rewrote the chapter a bit, as it was pretty confusing. In the listener, the client data "event" was used to update the form in PRE_BIND, but in the example this field wasn't even added to the form, making the example broken.

cc @bschussek