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/vty/command.c b/src/vty/command.c
index 587bd62..e50706e 100644
--- a/src/vty/command.c
+++ b/src/vty/command.c
@@ -42,7 +42,8 @@
 #include <osmocom/core/utils.h>
 
 /*! \addtogroup command
- * @{
+ *  @{
+ *  \brief VTY command handling
  */
 /*! \file command.c */