core/cib: fix documented maximum size restrictions
core/cib: fix documented maximum size restrictions

MAXINT is not a defined value and since the types are unsigned also
misleading.

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