Document how to check online status
In order to get more to higher level APIs it might be good to document how to check for online status via the nm APIs (see e.g. chatty#92 (closed)). Maybe listening for Airplaine mode is also interesting: https://gitlab.gnome.org/GNOME/gnome-settings-daemon/blob/master/plugins/rfkill/gsd-rfkill-manager.c