minor #7514 Update session_configuration.rst (MicWit, javiereguiluz)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7514).

Discussion
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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Required changes to garbage collection documentation as it was out of date.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.

Commits
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7b9a3e5 Minor rewords
4dc0dac Update session_configuration.rst

Author: xabbuh

components/http_foundation/session_configuration.rst

@@ -139,6 +139,20 @@ 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, some operating systems do their own session handling and set the
+``session.gc_probability`` variable to ``0`` to stop PHP doing garbage
+collection. That's why Symfony now overwrites this value to ``1``.
+
+If you wish to use the original value set in your ``php.ini``, 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`