update/extend doxygen documentation

It's a pity that even with this patch we still are fare away from having
the whole API documented.  However, at least we have a more solid
foundation.  Updates not only extend the documentation, but also make
sure it is rendered properly in the doxygen HTML.

Change-Id: I1344bd1a6869fb00de7c1899a8db93bba9bafce3
diff --git a/src/gb/gprs_ns.c b/src/gb/gprs_ns.c
index 23c0be8..17933ed 100644
--- a/src/gb/gprs_ns.c
+++ b/src/gb/gprs_ns.c
@@ -1648,4 +1648,4 @@
 	DNS = ss;
 }
 
-/*! }@ */
+/*! @} */