Update session_configuration.rst

Required changes to garbage collection documentation as it was out of date. Note that this is my first time editing and I had a look at the instructions, but some things may not be quite correct as the preview didn't seem to work properly. I didn't download the files and edit, I used the online editor. I also wasn't sure about links to other parts of the doc, but may be worth having a link to http://symfony.com/doc/current/reference/configuration/framework.html#gc-probability where I edited.

Author: MicWit

components/http_foundation/session_configuration.rst

@@ -139,6 +139,17 @@ the ``php.ini`` directive ``session.gc_maxlifetime``. The meaning in this contex
 that any stored session that was saved more than ``gc_maxlifetime`` ago should be
 deleted. This allows one to expire records based on idle time.
 
+However, because some operating systems do their own session handling and have set the ``session.gc_probability`` variable to 0 (to stop PHP doing garbage collection), Symfony now overwrites this value to a value of 1.
+
+If you wish to use the value set in your PHP.ini, you would need to add the following configuration:
+
+.. code-block:: yaml
+
+# config.yml
+framework:
+    session:
+        gc_probability: null
+
 You can configure these settings by passing ``gc_probability``, ``gc_divisor``
 and ``gc_maxlifetime`` in an array to the constructor of
 :class:`Symfony\\Component\\HttpFoundation\\Session\\Storage\\NativeSessionStorage`