Mention translations are not covered by BC promise

[OskarStark]

Sure, I understand that. Still, I don't think this should be considered as a BC break as we're adding translations on 6.4 just to ease our work.

Works for me, I agree with Oskar though that it might make sense to add this to our BC promise.

Originally posted by @xabbuh in symfony/symfony#60486 (comment)