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/gsm0411_smc.c b/src/gsm/gsm0411_smc.c
index 4c08365..03016b9 100644
--- a/src/gsm/gsm0411_smc.c
+++ b/src/gsm/gsm0411_smc.c
@@ -60,6 +60,11 @@
 #include <osmocom/gsm/gsm0411_smc.h>
 #include <osmocom/gsm/protocol/gsm_04_08.h>
 
+/*! \addtogroup sms
+ *  @{
+ *  \brief Point-to-Point (PP) Short Message Service (SMS) as per TS 04.11
+ */
+
 static void cp_timer_expired(void *data);
 
 #define MAX_SMS_RETRY 2
@@ -569,3 +574,5 @@
 
 	return rc;
 }
+
+/*! @} */