[doc] make sure all SMS related code is part of the 'sms' group

Change-Id: I24c56ccb56d5b39cfb887808f91b715da54c0f8b
diff --git a/include/osmocom/gsm/gsm0411_smr.h b/include/osmocom/gsm/gsm0411_smr.h
index 19f9c5d..28f43de 100644
--- a/include/osmocom/gsm/gsm0411_smr.h
+++ b/include/osmocom/gsm/gsm0411_smr.h
@@ -1,4 +1,6 @@
-/*! \file gsm0411_smr.h */
+/*! \addtogroup sms
+ *  @{
+ * \file gsm0411_smr.h */
 
 #pragma once
 
@@ -42,3 +44,5 @@
 /* message from lower layer */
 int gsm411_smr_recv(struct gsm411_smr_inst *inst, int msg_type,
 	struct msgb *msg);
+
+/*! @} */