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/serial.c b/src/serial.c
index 05bdc86..51a32bb 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -24,11 +24,10 @@
 
 /*! \addtogroup serial
  *  @{
+ *  \brief Osmocom serial port helpers
  */
 
-/*! \file serial.c
- * Osmocom serial port helpers
- */
+/*! \file serial.c */
 
 #include <errno.h>
 #include <fcntl.h>