Mention session.gc_* changes in 8.4 to fix #4421

Author: philip

appendices/migration84/incompatible.xml

@@ -359,6 +359,16 @@
    </para>
   </sect3>
 
+  <sect3 xml:id="migration84.incompatible.new-warnings-exceptions.session">
+   <title>Session</title>
+
+   <simpara>
+    Setting a non-positive value for <link linkend="ini.session.gc-divisor">session.gc_divisor</link>
+    or a negative value for <link linkend="ini.session.gc-probability">session.gc_probability</link>
+    now emits a warning.
+   </simpara>
+  </sect3>
+
   <sect3 xml:id="migration84.incompatible.new-warnings-exceptions.simplexml">
    <title>SimpleXML</title>