registrar: docs for event_callback parameter

src/modules/registrar/doc/registrar_admin.xml

@@ -961,6 +961,37 @@ modparam("registrar", "contact_max_size", 1024)
 		</example>
 	</section>
 
+	<section id="registrar.p.event_callback">
+		<title><varname>event_callback</varname> (str)</title>
+		<para>
+			The name of the function in the KEMI configuration file (embedded
+			scripting language such as Lua, Python, ...) to be executed instead
+			of event_route[...] blocks.
+		</para>
+		<para>
+			The function receives a string parameter with the name of the event.
+            The only possible value currently is 'usrloc:contact-expired'.
+		</para>
+		<para>
+		<emphasis>
+			Default value is 'empty' (no function is executed for events).
+		</emphasis>
+		</para>
+		<example>
+		<title>Set <varname>event_callback</varname> parameter</title>
+		<programlisting format="linespecific">
+...
+modparam("registrar", "event_callback", "ksr_registrar_event")
+...
+-- event callback function implemented in Lua
+function ksr_registrar_event(evname)
+    KSR.info( "Expired contact for " .. KSR.pv.getw("$ulc(exp=>aor)") .. "\n");
+	return 1;
+end
+...
+        </programlisting>
+		</example>
+	</section>
 	</section>