presence_b2b documentation added to "presence handbook"

doc/presence/presence_book.xml

@@ -17,6 +17,7 @@ http://www.sagehill.net/docbookxsl/ValidXinclude.htm -->
 	<section><title>Presence modules</title>
 	<include xmlns="http://www.w3.org/2001/XInclude" href="../../modules/pa/doc/pa.xml"/>
 	<include xmlns="http://www.w3.org/2001/XInclude" href="../../modules/rls/doc/rls.xml"/>
+	<include xmlns="http://www.w3.org/2001/XInclude" href="../../modules/presence_b2b/doc/presence_b2b.xml"/>
 	</section>
 	<include xmlns="http://www.w3.org/2001/XInclude" href="examples.xml"/>
 	<include xmlns="http://www.w3.org/2001/XInclude" href="biblio.xml"/>