[Translation] [2.6] Upgrade information for LoggingTranslator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

When upgrading a Symfony 2.5 project to the 2.6 branch, I noticed that the `@translator` service was changed. In the affected project, we've had a service that depends on the `@translator` service and uses a type hint to ensure that a `Translator` instance is passed to the constructor. With the introduction of the `LoggingTranslator` class (PR #10887), this type hint now fails.

I have added a small note to ` UPGRADE-2.6.md` in case more people stumble across this changed behavior.