feature #7921 Adding an article to explain the 3.3 changes, and how to upgrade (weaverryan)

This PR was submitted for the master branch but it was merged into the 3.3 branch instead (closes #7921).

Discussion
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Adding an article to explain the 3.3 changes, and how to upgrade

This article is aimed at existing Symfony users: I want to explain how the new DI features work and the "why" behind them a bit more (and at a deeper technical level).

I'll also write a blog post on symfony.com pointing to this article.

Review - related to how things are explained, clarity, technical points, etc - is quite needed! :)

Thanks!

Commits
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8bbecf2 Adding an article to explain the 3.3 changes, and how to upgrade

Author: weaverryan

controller.rst

@@ -142,6 +142,8 @@ Add the ``use`` statement atop the ``Controller`` class and then modify
 That's it! You now have access to methods like :ref:`$this->render() <controller-rendering-templates>`
 and many others that you'll learn about next.
 
+.. _controller-abstract-versus-controller:
+
 .. tip::
 
     You can extend either ``Controller`` or ``AbstractController``. The difference

controller/service.rst

@@ -152,6 +152,8 @@ the route ``_controller`` value:
     fully-qualified class name (FQCN). See the
     `FrameworkExtraBundle documentation`_ for details.
 
+.. _controller-service-invoke:
+
 .. tip::
 
     If your controller implements the ``__invoke()`` method, you can simply

service_container.rst

@@ -117,6 +117,11 @@ in the container.
 Creating/Configuring Services in the Container
 ----------------------------------------------
 
+.. tip::
+
+    The recommended way of configuring services changed in Symfony 3.3. For a deep
+    explanation, see :doc:`/service_container/3.3-di-changes`.
+
 You can also organize your *own* code into services. For example, suppose you need
 to show your users a random, happy message. If you put this code in your controller,
 it can't be re-used. Instead, you decide to create a new class::