Notes about caching pages with a CSRF Form

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | #1216
I just noticed this got deprecated in 2.4... too bad since it is very concise to explain it.

Should we add different code for 2.3 and the rest of Symfony versions?