Notes about caching pages with a CSRF Form

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | #1216
Unless you want to spend some time on the extension of the book chapter, I'd finish this pull request first. This can then be reworded or reorganised later on when someone is working on #4151.