bug #8426 Property class is required (alexpts)

This PR was squashed before being merged into the 3.3 branch (closes #8426).

Discussion
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Property class is required

Without property `class` throw exception:
`Uncaught Symfony\Component\DependencyInjection\Exception\RuntimeException: Please add the class to service "$name" even if it is constructed by a factory since we might need to add method calls based on compile-time checks.`

Commits
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f8f0e30 Property class is required

Author: wouterj

service_container/factories.rst

@@ -153,6 +153,7 @@ Configuration of the service container then looks like this:
         # app/config/services.yml
 
         app.newsletter_manager:
+            class: AppBundle\Email\NewsletterManager 
             # new syntax
             factory: 'AppBundle\Email\NewsletterManagerFactory:createNewsletterManager'
             # old syntax