In the latest kernel errata for SL 40,41,42 i386,x86_64, kernel-doc is mentioned:
kernel-doc-2.6.9-22.0.2.EL.noarch.rpm
However this does not exist in the i386 SL 4.2 errata rpms. Just for verification, I
checked and found out that CentOS has it.
I haven't checked whether other kernel rpms are missing.