OsmoMGW: document the 'X-Osmo-IGN' MGCP extension

See also Ia6fe5ead4b601931c1bf41b29fc1b237aac37d2c in osmo-mgw.git, which
changes the initial single-character implementation to match this specification
of string tokens separated by spaces.

Change-Id: If15a88c3b5b40fd1d24ad0f94f3231f678669ab1
diff --git a/doc/manuals/osmomgw-usermanual.adoc b/doc/manuals/osmomgw-usermanual.adoc
index 69dc005..6030023 100644
--- a/doc/manuals/osmomgw-usermanual.adoc
+++ b/doc/manuals/osmomgw-usermanual.adoc
@@ -18,6 +18,8 @@
 
 include::chapters/configuration.adoc[]
 
+include::chapters/mgcp_extensions.adoc[]
+
 //include::chapters/counters.adoc[]
 
 include::../common/chapters/port_numbers.adoc[]