Added a note about inlined private services
Emphasized the fact that this only applies when the private service is used as argument for a single service
added the title of a cross reference
minor #3727 Added a note about inlined private services (javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3727).

Discussion
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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
```

Commits
-------

15a7be0 [book] [service_container] added the title of a cross reference
835b52f Emphasized the fact that this only applies when the private service is used as argument for a single service
04cb9d4 Added a note about inlined private services