[DI] Documentation for the autowire() shortcut method

[xabbuh]

The new autowire() method of the ContainerBuilder that was introduced in symfony/symfony#20648 is a shortcut for $container->setDefinition($id, (new Definition($class))->setAutowired(true)).

[wouterj]

This method can be documented in http://symfony.com/doc/current/service_container/definitions

The method can also be document and used in the PHP code examples in http://symfony.com/doc/current/components/dependency_injection/autowiring.html

[GuilhemN]

see #7858