Added a note about the side effects of enabling both PHP and Twig

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #3293
This should be a bit more precise. They will still work in places where you talk to Twig directly (for instance all places referencing a template from another one)