boards/slstk3401a: update documentation
boards/slstk3401a: update documentation

ant9000/RIOT,x3ro/RIOT,yogo1212/RIOT,miri64/RIOT,toonst/RIOT,mtausig/RIOT,x3ro/RIOT,ant9000/RIOT,OTAkeys/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,ant9000/RIOT,toonst/RIOT,kaspar030/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,x3ro/RIOT,jasonatran/RIOT,miri64/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,basilfx/RIOT,OTAkeys/RIOT,mtausig/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OTAkeys/RIOT,yogo1212/RIOT,kYc0o/RIOT,smlng/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,mtausig/RIOT,toonst/RIOT,mtausig/RIOT,smlng/RIOT,miri64/RIOT,basilfx/RIOT,mtausig/RIOT,OlegHahm/RIOT,toonst/RIOT,authmillenon/RIOT,basilfx/RIOT,smlng/RIOT,jasonatran/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,yogo1212/RIOT,smlng/RIOT,toonst/RIOT,kYc0o/RIOT,x3ro/RIOT,kaspar030/RIOT,kYc0o/RIOT,x3ro/RIOT,jasonatran/RIOT,kYc0o/RIOT,basilfx/RIOT,basilfx/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,smlng/RIOT