sys/checksum: update CRC-16 documentation
sys/checksum: update CRC-16 documentation

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