[document][leo_object_storage] objects may be inaccessible after rollback previous version

[mocchira]

We've already had kinda automatic meta|object migration logic in https://github.com/leo-project/leo_object_storage/blob/develop/src/leo_object_storage_transformer.erl so basically users don't need to take care even if meta|object format changed because leofs automatically handle migrations behind the scene. but there is one pitfall what if users faced some critical issue and need to rollback their leofs to previous one? objects that already got transformed into a new format will be inaccessible after rollback. we may need some solution for that.