Reworded the explanation about service tags

Author: javiereguiluz

service_container/tags.rst

@@ -5,10 +5,15 @@
 How to Work with Service Tags
 =============================
 
-In the same way that a blog post on the web might be tagged with things such
-as "Symfony" or "PHP", services configured in your container can also be
-tagged. In the service container, a tag implies that the service is meant
-to be used for a specific purpose. Take the following example:
+Some container services require to be registered or used in some special way.
+Consider for example the :doc:`Twig extensions </templating/twig_extension>`:
+they are PHP classes that must be added to the main Twig object before using
+them. If you define those exensions as regular Symfony services, Twig will
+ignore them and an exception will be thrown when using them in templates.
+
+**Service tags** are a way to tell Symfony or other third-party bundles that
+your service should be registered in some special way. Take the following
+example:
 
 .. configuration-block::
 
@@ -51,14 +56,10 @@ to be used for a specific purpose. Take the following example:
             ->setPublic(false)
             ->addTag('twig.extension');
 
-The ``twig.extension`` tag is a special tag that the TwigBundle uses
-during configuration. By giving the service this ``twig.extension`` tag,
-the bundle knows that the ``app.twig_extension`` service should be registered
-as a Twig extension with Twig. In other words, Twig finds all services tagged
-with ``twig.extension`` and automatically registers them as extensions.
-
-Tags, then, are a way to tell Symfony or other third-party bundles that
-your service should be registered or used in some special way by the bundle.
+Services tagged with the ``twig.extension`` tag are collected during the
+initialization of TwigBundle to enable them as Twig extensions. In this example,
+that's why Symfony knows that the ``app.twig_extension`` service should be
+registered as a Twig extension.
 
 For a list of all the tags available in the core Symfony Framework, check
 out :doc:`/reference/dic_tags`. Each of these has a different effect on your