Hello Ravi !
I'm able to see in ECC 6( Software component : SAP_BASIS ; Release : 731 ; Level : 0006), the standard user decision task comes with reason for rejection under "Justification" column as depicted below :
If you're on lower release, either ask your BASIS team to apply patches for above release or use the function module CATSXT_SIMPLE_TEXT_EDITOR in rejection branch to enter reason for rejection.
Regards,
S.Suresh.