UserManual: Include sigtran*.adoc from osmo-gsm-manuals.git

They were only included in osmo-stp so far.

Change-Id: I37e36e7919d6996a79533b62564c5d82908b57f7
diff --git a/doc/manuals/osmomsc-usermanual.adoc b/doc/manuals/osmomsc-usermanual.adoc
index 632abc7..ad55e53 100644
--- a/doc/manuals/osmomsc-usermanual.adoc
+++ b/doc/manuals/osmomsc-usermanual.adoc
@@ -22,6 +22,10 @@
 
 include::./common/chapters/logging.adoc[]
 
+include::./common/chapters/sigtran-osmocom.adoc[]
+
+include::./common/chapters/cs7-config.adoc[]
+
 include::./common/chapters/cs7-config.adoc[]
 
 include::{srcdir}/chapters/net.adoc[]