[DependencyInjection] Add docs for SubscribedService::$attributes

[kbond]

Fixes #17208.

[kbond]

One potential problem with this feature is it will require the latest version of symfony/service-contracts (3.2+) but symfony/dependency-injection currently allows older versions.

Basically, a user could have symfony/dependency-injection 6.2 but symfony/service-contracts 3.1. This feature will not work in this case.

[mdoutreluingne]

I think it would be interesting here to mention that this feature requires symfony/service-contracts (3.2+), because in your example there may be a case where this feature would not work depending on the version of symfony/service-contracts.

[javiereguiluz]

Thanks Kevin! Maxime, I added that note to both examples while merging (see 1aa135c).