[DependencyInjection] Update tags.rst

[axi]

I think it's the new recommanded way for bundles (as seen here https://symfony.com/doc/current/bundles/extension.html)

[javiereguiluz]

I like this a lot. Thanks Alexis.

While merging, I removed the old way of doing this (and replace it by a minor note comment) to only show the new method that you added. See 7b9ffb2