Added a note about inlined private services

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

When you define private services that are used just as arguments for another service, Symfon2 doesn't define them as services, but inlines them in the other service definition. This slightly improves the performance of the container, but it's confusing for Symfony beginners because the private services aren't displayed by the `container:debug` command even when using the `--show-private` option:

```
$ php app/console container:debug --show-private
```