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/fsm.c b/src/fsm.c
index 5e74482..fbe22d7 100644
--- a/src/fsm.c
+++ b/src/fsm.c
@@ -29,9 +29,6 @@
 
 /*! \addtogroup fsm
  *  @{
- */
-
-/*! \file fsm.c
  *  \brief Finite State Machine abstraction
  *
  *  This is a generic C-language abstraction for implementing finite
@@ -84,9 +81,10 @@
  *
  *  In order to attach private state to the \ref osmo_fsm_inst, it
  *  offers an opaque priv pointer.
- *
  */
 
+/*! \file fsm.c */
+
 LLIST_HEAD(osmo_g_fsms);
 static bool fsm_log_addr = true;