doc/kconfig: add information on default configurations
doc/kconfig: add information on default configurations

kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT