Added a section about the framework

cookbook/upgrade/deprecation_warnings.rst

@@ -20,7 +20,26 @@ How can I Silence the Warnings?
 
 As useful as these are, you don't want them to show up while developing and
 you may also want to silence them on production to avoid filling up your
-error logs. To do that, add ``~E_USER_DEPRECATED`` to your ``error_reporting``
+error logs.
+
+In the Symfony Framework
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+In the Symfony Framework, ``~E_USER_DEPRECATED`` is added to ``app/bootstrap.php.cache``
+automatically, but you need at least version 2.3.14 or 3.0.21 of the
+`SensioDistributionBundle`_. So, you may need to upgrade:
+
+.. code-block:: bash
+
+    composer update sensio/distribution-bundle
+
+Once you've updated, the ``bootstrap.php.cache`` file is rebuilt automatically.
+At the top, you should see a line adding ``~E_USER_DEPRECATED``.
+
+Outside of the Symfony Framework
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To do that, add ``~E_USER_DEPRECATED`` to your ``error_reporting``
 setting in ``php.ini``:
 
 .. code-block:: ini
@@ -52,4 +71,4 @@ Once all the deprecation warnings are gone, you can upgrade with a lot
 more confidence.
 
 .. _`phpunit-bridge`: https://github.com/symfony/phpunit-bridge
-
+.. _`SensioDistributionBundle`: https://github.com/sensiolabs/SensioDistributionBundle