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 will be very vague for most users. I prefer something like "The `@` notation for twig namespaces will no longer be supported for the `render()` method."