app_ruby: docs for xval_mode parameter

src/modules/app_ruby/doc/app_ruby_admin.xml

@@ -85,7 +85,37 @@ modparam("app_ruby", "load", "/usr/local/etc/kamailio/ruby/myscript.rb")
 </programlisting>
 	    </example>
 	</section>
-
+	<section id="app_ruby.p.xval_mode">
+	    <title><varname>xval_mode</varname> (int)</title>
+	    <para>
+			Control if the external sub-module functions returning extended-values
+			should propagate their string return value (when set to 0) or be
+			replaced by NULL/nil (when set to 1).
+			</para>
+			<para>
+			When set to 0, the KSR::PV Ruby submodule is implemented with the
+			internal functions from the app_ruby module, otherwise the ones from
+			core are used.
+	    </para>
+	    <para>
+			Note: when set to 1, there were crashing reports that are under
+			investigation, this option being provided as intermediary
+			solution to preserve the behaviour from older versions.
+	    </para>
+	    <para>
+		<emphasis>
+		    Default value is <quote>0</quote>.
+		</emphasis>
+	    </para>
+	    <example>
+		<title>Set <varname>xval_mode</varname> parameter</title>
+		<programlisting format="linespecific">
+...
+modparam("app_ruby", "xval_mode", 1)
+...
+</programlisting>
+	    </example>
+	</section>
 	</section>
 
     <section>