Notes about caching pages with a CSRF Form

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | #1216
This is tricky, because I want to address both Varnish and Symfony's HttpCache. I believe Varnish doesn't cache anything by default when there is a cookie present. But I believe that Symfony's HttpCache *does* cache these things. So maybe we need to talk more generically about *why* you can't cache these pages (and maybe mention that Varnish doesn't cache them by default for these reasons).