sys/luid: Added postcondition to luid_get()
sys/luid: Added postcondition to luid_get()

Stated the obvious in the doc: Altering the LUID returned by luid_get() breaks
the guarantee that it is locally unique, as it than could collide with other
LUIDs returned by luid_get().

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