bug symfony#3475 Fixed doc for framework.session.cookie_lifetime refr…

…ence. (tyomo4ka)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#3475).

Discussion
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Fixed doc for framework.session.cookie_lifetime refrence.

| Q           | A
| ------------ | ---
| Doc fix? | yes
| New docs? | no
| Applies to | 2.3+
| Fixed tickets |

Commits
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39a65e1 Fixed doc for framework.session.cookie_lifetime refrence.

reference/configuration/framework.rst

@@ -169,10 +169,12 @@ name which is defined in the ``php.ini`` with the ``session.name`` directive.
 cookie_lifetime
 ...............
 
-**type**: ``integer`` **default**: ``0``
+**type**: ``integer`` **default**: ``null``
 
-This determines the lifetime of the session - in seconds. By default it will use
-``0``, which means the cookie is valid for the length of the browser session.
+This determines the lifetime of the session - in seconds. It will use ``null`` by 
+default, which means ``session.cookie_lifetime`` value from ``php.ini`` will be used.
+Setting this value to ``0`` means the cookie is valid for the length of the browser 
+session.
 
 cookie_path
 ...........