Add some documentation for the callback

Author: Jarno Lamsa

features/netsocket/NetworkInterface.h

@@ -131,6 +131,10 @@ class NetworkInterface {
     virtual nsapi_error_t add_dns_server(const SocketAddress &address);
 
     /** Register callback for status reporting
+     *
+     *  The specified status callback function will be called on status changes
+     *  on the network. The parameters on the callback are the event type and
+     *  event-type dependent reason parameter.
      *
      *  @param status_cb The callback for status changes
      */