Update session.rst

May be a bug, but Sessions expire after 30 minutes (or less ?)
To have the default behavior describe in documentation, you must set cookie_lifetime to 0

$app->register(new Silex\Provider\UrlGeneratorServiceProvider(), array(
    'session.storage.options' => array(
        'cookie_lifetime' => 0
        )
    )
);
minor #995 Update session.rst (davtwix)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #995).

Discussion
----------

Update session.rst

May be a bug, but Sessions expire after 30 minutes (or less ?)
To have the default behavior describe in documentation, you must set cookie_lifetime to 0

$app->register(new Silex\Provider\UrlGeneratorServiceProvider(), array(
    'session.storage.options' => array(
        'cookie_lifetime' => 0
        )
    )
);

Commits
-------

0e553e3 Update session.rst