remove default value for gc_probability config option

Author: raphael-geffroy

reference/configuration/framework.rst

@@ -1852,6 +1852,8 @@ started on every session initialization. The probability is calculated by
 using ``gc_probability`` / ``gc_divisor``, e.g. 1/100 means there is a 1%
 chance that the GC process will start on each request.
 
+If not set, ``php.ini``'s `session.gc_probability`_ directive will be relied on.
+
 gc_maxlifetime
 ..............