Add config parameter to define session duration

docs/config.markdown

@@ -210,6 +210,15 @@ define('BRUTEFORCE_LOCKDOWN', 6);
 define('BRUTEFORCE_LOCKDOWN_DURATION', 15);
 ```
 
+Session
+-------
+
+```php
+// Session duration in second (0 = until the browser is closed)
+// See http://php.net/manual/en/session.configuration.php#ini.session.cookie-lifetime
+define('SESSION_DURATION', 0);
+```
+
 Various settings
 ----------------