I hesitated to use _defaults here to be able to remove this line and use the short syntax, wdyt?

I'd say, let's not add the "defaults" until we decide what to do with that new option.

To make versionadded a RST directive, you must include the two .. at the beginning: .. versionadded::

Indeed, thanks. I also fixed it a few lines above.

I think this should be moved to service_container, after #7807 is merged. I did not have that note there.

I agree I didn't realize I added it in tags.rst... I remove it and paste it here to not lose it.

.. tip::

    In YAML format, you may define a service with a simple array of tags as long
    as you don't need additional attributes. The following definitions are
    equivalent.

    .. code-block:: yaml

        services:

            # Compact syntax
            AppBundle\Twig\AppExtension: [twig.extension]

            # Verbose syntax
            AppBundle\Twig\AppExtension:
                tags: [twig.extension]

    .. versionadded:: 3.3
        Support for the short syntax for service definition in the YAML format
        was introduced in Symfony 3.3.

Actually rethinking about it, does it make sense? We don't talk about tags in service_container.

Don't we already have a place where we explain the difference?

I don't think so, it was requested by #7441.

Hm, not sure if we really need this.

Won't this be confusing for people who try to use it in lower versions because of our exemples using this syntax ?

Do we talk about different things? Do you mean the versionadded directive? Otherwise I don't see the problem as the changes were only merged into the master branch.