GNetworkMonitor: Rename "network-changed" signal argument

Because the argument being called "available" and the property being
called "network-available" is confusing.

Also remove the details of what that value means, as it's already
described in the property, and duplicating the explanation makes it look
like it might have a different meaning.

https://bugzilla.gnome.org/show_bug.cgi?id=792370

gio/gnetworkmonitor.h

@@ -48,7 +48,7 @@ struct _GNetworkMonitorInterface {
   GTypeInterface g_iface;
 
   void     (*network_changed)  (GNetworkMonitor      *monitor,
-				gboolean              available);
+				gboolean              network_available);
 
   gboolean (*can_reach)        (GNetworkMonitor      *monitor,
 				GSocketConnectable   *connectable,