registrar: documented lookup_to_dset() cfg function [skip ci]

- function available for long time, but somehow was not documented
- GH #2623

src/modules/registrar/doc/registrar_admin.xml

@@ -1338,7 +1338,8 @@ save("location", "0x00", "sip:[email protected]");
 		<programlisting format="linespecific">
 ...
 lookup("location");
-switch ($retcode) {
+			switch ($retcode) {:1
+
     case -1:
     case -3:
         sl_send_reply("404", "Not Found");
@@ -1352,6 +1353,29 @@ switch ($retcode) {
 		</example>
 	</section>
 
+	<section id="registrar.f.lookup_to_dset">
+		<title>
+		<function moreinfo="none">lookup_to_dset(domain [, uri])</function>
+		</title>
+		<para>
+			Similar to lookup(...), but push the location contacts to destination
+			set, without changing the R-URI (first branch not changed, it creates
+			additional branches). For the meaning of the parameters and the return
+			codes, see the documentation for lookup(...) function.
+		</para>
+		<para>
+		This function can be used from REQUEST_ROUTE, FAILURE_ROUTE.
+		</para>
+		<example>
+		<title><function>lookup_to_dset</function> usage</title>
+		<programlisting format="linespecific">
+...
+lookup_to_dset("location");
+...
+</programlisting>
+		</example>
+	</section>
+
 	<section id="registrar.f.lookup_branches">
 		<title>
 		<function moreinfo="none">lookup_branches(domain)</function>