Document the new short syntax for factory declarations

See https://github.com/symfony/symfony/pull/20943

Before:

```xml
<service id="app.newsletter_manager" class="AppBundle\Email\NewsletterManager">
    <factory class="AppBundle\Email\NewsletterManager" method="create" />
</service>
```

After:

```xml
<service id="app.newsletter_manager" class="AppBundle\Email\NewsletterManager">
    <factory method="create" />
</service>
```