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/bitvec.c b/src/bitvec.c
index c895cff..41e4f64 100644
--- a/src/bitvec.c
+++ b/src/bitvec.c
@@ -24,12 +24,11 @@
 
 /*! \addtogroup bitvec
  *  @{
- */
-
-/*! \file bitvec.c
  *  \brief Osmocom bit vector abstraction
  */
 
+/*! \file bitvec.c */
+
 #include <errno.h>
 #include <stdint.h>
 #include <string.h>