[DependencyInjection] Add documentation about service decoration

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#10600)
| Applies to    | 2.5+
| Fixed tickets | #3745
Yes, in fact, why not use a real service from the framework? Sure, this is the components, but *any* service we choose here will be "invented", so why not use on that's a real service somewhere.