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_comp128v23.c b/src/gsm/auth_comp128v23.c
index 168f886..b0900af 100644
--- a/src/gsm/auth_comp128v23.c
+++ b/src/gsm/auth_comp128v23.c
@@ -27,6 +27,10 @@
 #include <osmocom/crypt/auth.h>
 #include <osmocom/gsm/comp128v23.h>
 
+/*! \addtogroup auth
+ *  @{
+ */
+
 static int c128v2_gen_vec(struct osmo_auth_vector *vec,
 			  struct osmo_sub_auth_data *aud,
 			  const uint8_t *_rand)
@@ -66,3 +70,5 @@
 	osmo_auth_register(&c128v2_alg);
 	osmo_auth_register(&c128v3_alg);
 }
+
+/*! @} */