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_smr.c b/src/gsm/gsm0411_smr.c
index eb8aec0..d120f2d 100644
--- a/src/gsm/gsm0411_smr.c
+++ b/src/gsm/gsm0411_smr.c
@@ -62,6 +62,10 @@
 
 #define SMR_LOG_STR "SMR(%" PRIu64 ") "
 
+/*! \addtogroup sms
+ *  @{
+ */
+
 static void rp_timer_expired(void *data);
 
 /* init a new instance */
@@ -484,3 +488,5 @@
 
 	return rc;
 }
+
+/*! @} */