[FrameworkBundle] Fix cookie_secure default value

Author: Mathieu

reference/configuration/framework.rst

@@ -1704,12 +1704,15 @@ The possible values for this option are:
 cookie_secure
 .............
 
-**type**: ``boolean`` or ``'auto'`` **default**: ``'auto'``
+**type**: ``boolean`` or ``'auto'`` **default**: ``null``
 
 This determines whether cookies should only be sent over secure connections. In
 addition to ``true`` and ``false``, there's a special ``'auto'`` value that
 means ``true`` for HTTPS requests and ``false`` for HTTP requests.
 
+By default, it will use the value defined in the ``php.ini`` with the
+``session.cookie_secure`` directive.
+
 cookie_httponly
 ...............