[FrameworkBundle][config] added versionadded + label for the old name.

Author: aitboudad

reference/configuration/framework.rst

@@ -526,9 +526,16 @@ enabled
 
 Whether or not to enable the ``translator`` service in the service container.
 
+.. _fallback:
+
 fallbacks
 .........
 
+.. versionadded:: 2.3.25
+    The ``fallbacks`` option was introduced in Symfony 2.3.25. Prior
+    to Symfony 2.3.25, it was called ``fallback`` and only allowed one
+    fallback language defined as a string.
+
 **type**: ``string|array`` **default**: ``en``
 
 This option is used when the translation key for the current locale wasn't found.