[WaitingCodeMerge][Book][ServiceContainer] Added new 2.3 feature '@@' escaping

Added version note for 2.3 about the new @@ escaping strategy
introduced in symfony/symfony#7307

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes https://github.com/symfony/symfony/pull/7307
| Applies to    | 2.3
| Fixed tickets | https://github.com/symfony/symfony/issues/4889