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/gsm/lapdm.c b/src/gsm/lapdm.c
index 99099d1..8ec942f 100644
--- a/src/gsm/lapdm.c
+++ b/src/gsm/lapdm.c
@@ -23,6 +23,7 @@
 
 /*! \addtogroup lapdm
  *  @{
+ *  \brief GSM LAPDm (TS 04.06) implementation
  */
 
 /*! \file lapdm.c */