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.c b/src/logging.c
index 0d2b6da..3b3dd05 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -22,6 +22,7 @@
 
 /* \addtogroup logging
  * @{
+ * \brief libosmocore Logging sub-system
  */
 
 /* \file logging.c */