[WCM] [The Book, Translations] added doc for translation domain cascading

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8489)
| Applies to    | all (or 2.3+)

Documentation for an unmerged PR which adds the possibility to use an array of translation domains instead of a single one, thus providing a fallback mechanism.