Add documentation for anonymous services

[Haehnchen]

Do we have any documentation for "anonymous services"? i googled some terms about this feature but not getting results on the official page. Some improvements would be nice here. :)

related: Haehnchen/idea-php-symfony2-plugin#595

The XML format supports anonymous services. An anonymous service is a service that does not need a name and is defined directly in its use context. It can be very convenient when you need a service that won't be used outside of another one scope:
http://fabien.potencier.org/symfony-service-container-using-xml-or-yaml-to-describe-services.html

[xabbuh]

I am not sure if we should really document this. Imho it's only useful for the dumpers to be able to dump the optimised container. But as the end user you don't really need this feature as making services private leads to the same anyway when the container is compiled.

[xabbuh]

And by the way, the Yaml format does not support anonymous services and won't get this feature in the future (see symfony/symfony#16328).

[HeahDude]

@xabbuh So may be this one should get closed.

[Oliboy50]

@xabbuh You're right, but this is maybe the only "syntactic sugar" for XML to compensate for its insane verbosity... It just seems sad to not document this

[SimonHeimberg]

When "anonymous service" is undocumented, it should not be mentioned in the doc in http://symfony.com/doc/current/service_container/service_locators.html#defining-a-service-locator

[Haehnchen]

now it should be documented; also mentioned in #7694, see also symfony/symfony#21970

[HeahDude]

Reopening until all versions and formats are properly documented.