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/conv.c b/src/conv.c
index 79b3a7c..8e97e52 100644
--- a/src/conv.c
+++ b/src/conv.c
@@ -24,11 +24,10 @@
 
 /*! \addtogroup conv
  *  @{
+ *  \brief Osmocom convolutional encoder and decoder
  */
 
-/*! \file conv.c
- * Osmocom convolutional encoder and decoder
- */
+/*! \file conv.c */
 #include "config.h"
 #ifdef HAVE_ALLOCA_H
 #include <alloca.h>