boards/feather-m0: update doc about flash and stdio
boards/feather-m0: update doc about flash and stdio

yogo1212/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,OTAkeys/RIOT,basilfx/RIOT,ant9000/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,yogo1212/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,ant9000/RIOT,ant9000/RIOT,basilfx/RIOT,yogo1212/RIOT,OlegHahm/RIOT,yogo1212/RIOT,kYc0o/RIOT,basilfx/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,basilfx/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,kaspar030/RIOT,yogo1212/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT