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/logging_syslog.c b/src/logging_syslog.c
index 5b0ae5f..6fe3c71 100644
--- a/src/logging_syslog.c
+++ b/src/logging_syslog.c
@@ -23,7 +23,9 @@
  *  @{
  */
 
-/*! \file logging_syslog.c */
+/*! \file logging_syslog.c
+ *  \brief libosmocore logging output via syslog
+ */
 
 #include "../config.h"