keepalove: added notes in docs about how monitoring is done

- closes GH #1082

src/modules/keepalive/doc/keepalive_admin.xml

@@ -10,13 +10,21 @@
 <!-- Module User's Guide -->
 
 <chapter>
-	
+
 	<title>&adminguide;</title>
-	
+
 	<section>
 		<title>Overview</title>
 		<para>
-		This module performs destinations monitoring either for itself, or on the behalf of other modules
+			This module performs destinations monitoring either for itself,
+			or on the behalf of other modules. The monitoring is done by sending
+			SIP OPTIONS requests, more or less in the same fashion as the
+			dispatcher module (which was the initial source for this module).
+		</para>
+		<para>
+			As an example of usage by other modules, see
+			<emphasis>drouting</emphasis>, which was
+			enahnced to use this module to monitor its gateways.
 		</para>
 	</section>