doc/manuals: re-organize description of internal USSD handlers
Change-Id: Ieddde02f3f41e0eb05b7223026da4252c17c3712
diff --git a/doc/manuals/chapters/ussd.adoc b/doc/manuals/chapters/ussd.adoc
index 1d962b7..3ab587f 100644
--- a/doc/manuals/chapters/ussd.adoc
+++ b/doc/manuals/chapters/ussd.adoc
@@ -52,13 +52,25 @@
`ussd route prefix *#100# internal own-msisdn` installs a prefix route
to the named internal USSD handler. The above command will restore
the old behavior, in which *#100# will return a text message containing
-the subscribers own phone number. There is one other handler called
-`own-imsi` which will return the IMSI instead of the MSISDN.
+the subscribers own phone number. More information on internal USSD
+handlers can be found in <<iuse_handlers>>.
`ussd default-route external foobar-00-00-00-00-00-00` installs a
default route to the named EUSE. This means that all USSD codes for
which no more specific route exists will be routed to the named EUSE.
+[[iuse_handlers]]
+=== Built-in USSD handlers
+
+OsmoHLR has an Internal USSD Entity (IUSE) that allows to handle some
+USSD requests internally. It features a set of simple handlers, which
+can be assigned to one or more USSD request prefixes:
+
+* `own-msisdn` returns subscriber's MSISDN (if assigned);
+* `own-imsi` returns subscriber's IMSI.
+
+Additional handlers can be added on request.
+
=== Example EUSE program
We have provided an example EUSE developed in C language using existing