Reworded the explanation

Author: javiereguiluz

templating.rst

@@ -365,9 +365,6 @@ template living inside the AcmeBlogBundle, for example, can be overridden
 by placing a template of the same name in the ``app/Resources/AcmeBlogBundle/views/``
 directory. This gives the power to override templates from any vendor bundle.
 
-If you have overridden a template, you can use the "!" prefix to refer to the original bundle's
-template. E.g. ``@!AcmeBlog/layout.html.twig``
-
 Template Suffix
 ~~~~~~~~~~~~~~~
 

templating/overriding.rst

@@ -19,11 +19,25 @@ from the bundle to ``app/Resources/AcmeBlogBundle/views/Blog/index.html.twig``
 (the ``app/Resources/AcmeBlogBundle`` directory won't exist, so you'll need
 to create it). You're now free to customize the template.
 
+Instead of overriding an entire template, you may just want to override one or
+more blocks. However, since you are overriding the template you want to extend
+from, you would end up in an infinite loop error. The solution is to use the
+special ``!`` prefix in the template name to tell Symfony that you want to
+extend from the original template, not from the overridden one:
+
+.. code-block:: twig
+
+    {# app/Resources/AcmeBlogBundle/views/Blog/index.html.twig #}
+
+    {# the special '!' prefix avoids errors when extending from an overridden template #}
+    {% extends "@!AcmeBlogBundle/index.html.twig" %}
+
+    {% block some_block %}
+        ...
+    {% endblock %}
+
 .. versionadded:: 3.4
-  
-  Instead of overridding an entire template, you may just want to override one or more blocks. You can do that by extending the original
-  template using the "!" prefix. For example:
-  ``{% extends "@!AcmeBlogBundle/layout.html.twig" %}``
+    The special ``!`` template prefix was introduced in Symfony 3.4.
 
 .. caution::