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/gan.c b/src/gsm/gan.c
index 0bcaa17..1d98b38 100644
--- a/src/gsm/gan.c
+++ b/src/gsm/gan.c
@@ -21,6 +21,10 @@
 
 #include <osmocom/gsm/protocol/gsm_44_318.h>
 
+/*! \file gan.c
+ *  \brief Generic Access Network (GAN) / UMA according to TS 44.318
+ */
+
 
 const struct value_string gan_msgt_vals[] = {
 	{ GA_MT_RC_DISCOVERY_REQUEST,	"GA-RC DISCOVERY REQUEST" },