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/gsm/ipa.c b/src/gsm/ipa.c
index fa6a8b0..5c37d64 100644
--- a/src/gsm/ipa.c
+++ b/src/gsm/ipa.c
@@ -41,6 +41,11 @@
 #include <osmocom/gsm/protocol/ipaccess.h>
 #include <osmocom/gsm/ipa.h>
 
+/*! \addtogroup ipa
+ *  @{
+ *  \brief IPA Multiplex utility routines
+ */
+
 #define IPA_ALLOC_SIZE 1200
 
 /*
@@ -581,3 +586,5 @@
 		return NULL;
 	return nmsg;
 }
+
+/*! @} */