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/write_queue.c b/src/write_queue.c
index c7a4320..1e4cda5 100644
--- a/src/write_queue.c
+++ b/src/write_queue.c
@@ -27,6 +27,7 @@
 
 /*! \addtogroup write_queue
  *  @{
+ *  \brief write queue for writing \ref msgb to sockets/fd's
  */
 
 /*! \file write_queue.c */