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/select.c b/src/select.c
index afc8198..bdd53b2 100644
--- a/src/select.c
+++ b/src/select.c
@@ -36,11 +36,10 @@
 
 /*! \addtogroup select
  *  @{
+ *  \brief select() loop abstraction
  */
 
-/*! \file select.c
- *  \brief select loop abstraction
- */
+/*! \file select.c */
 
 static int maxfd = 0;
 static LLIST_HEAD(osmo_fds);