[3.0] Form component documentation says to use classes SessionCsrfProvider and DefaultCsrfProvider, which were removed in 3.0

[OJezu]

In documentation of form component for 3.0 version the whole CSRF Protection is wrong:

http://symfony.com/doc/current/components/form/introduction.html#csrf-protection

Classes SessionCsrfProvider and DefaultCsrfProvider were removed from Form Component in 3.0
https://github.com/symfony/form/blob/2.8/Extension/Csrf/CsrfProvider/DefaultCsrfProvider.php#L14

[xabbuh]

Good catch @OJezu. In fact we need to update the document in the 2.7 branch to use the new classes from the Security CSRF component instead (the replacing classes are referenced in the deprecation notices of the legacy classes).

[xabbuh]

see #6077 for the pull request

[vincentaubert]

ping @xabbuh I have just rebased the branch with latest 2.7 changes see #6077. Is it ok for you to merge ?