Issue #12658 Add a deprecation note about Translator's setFallbackLocale...

... method
This should rather read like "The setFallbackLocale() method is deprecated since version 2.6 and will be removed in 3.0. Use setFallbackLocales() instead." (see http://symfony.com/doc/current/contributing/code/conventions.html#deprecations). You will probably have to change 2.6 to the version number where the `setFallbackLocales()` method was introduced.