cpu/atmega_common: cpuid: add a word of warning
cpu/atmega_common: cpuid: add a word of warning

The CPU ID only differs in byte 4 (RC calibration) between devices.

Add a word of warning to the documentation that this may not be very unique.

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