[WCM] Added depreciation note for the cascade_validation constraint

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6/all
| Fixed tickets | -

This ticket can be merged once symfony/symfony#12237 is merged into 2.6. 
I have moved the depreciation notes below the title so that you will see them with
a direct link. If you click http://symfony.com/doc/current/reference/forms/types/form.html#max-length
you will not see the depreciation note.

symfony/symfony-docs#4348


@WouterJ All files for constraints are written with an uppercased first letter. So, we should better change the reference here to point to `/reference/constraints/Valid`.