sys/fmt: add some documentation about `out`
sys/fmt: add some documentation about `out`

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