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/auth_milenage.c b/src/gsm/auth_milenage.c
index c1331b5..41e538d 100644
--- a/src/gsm/auth_milenage.c
+++ b/src/gsm/auth_milenage.c
@@ -25,6 +25,10 @@
 #include "milenage/common.h"
 #include "milenage/milenage.h"
 
+/*! \addtogroup auth
+ *  @{
+ */
+
 static int milenage_gen_vec(struct osmo_auth_vector *vec,
 			    struct osmo_sub_auth_data *aud,
 			    const uint8_t *_rand)
@@ -160,3 +164,5 @@
 {
 	osmo_auth_register(&milenage_alg);
 }
+
+/*! @} */