Expand the comment on the layout of softc, arpcom and ifnet structures,

and list the places where the assumption is used.

sys/net/if_var.h

@@ -110,6 +110,21 @@ struct	ifqueue {
 /*
  * NB: For FreeBSD, it is assumed that each NIC driver's softc starts with
  * one of these structures, typically held within an arpcom structure.
+ *
+ *	struct <foo>_softc {
+ *		struct arpcom {
+ *			struct  ifnet ac_if;
+ *			...
+ *		} <arpcom> ;
+ *		...
+ *	};
+ *
+ * The assumption is used in a number of places, including many
+ * files in sys/net, device drivers, and sys/dev/mii.c:miibus_attach().
+ *
+ * Unfortunately devices' softc are opaque, so we depend on this layout
+ * to locate the struct ifnet from the softc in the generic code.
+ * 
  */
 struct ifnet {
 	void	*if_softc;		/* pointer to driver state */