boards/stm32f4discovery: update documentation
boards/stm32f4discovery: update documentation

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