You don't have /usr/share/man/whatis because the catman -w command is failing. whatis is a man page database created by the catman -w command. The command may be failing because you might not have the file /usr/lbin/mkwhatis for some reason. Check to see if mkwhatis is there.
Use lslpp -l | grep bos.rte.man to see if you have the bos.rte.man fileset. Also, do an lslpp -f, pipe it to a file, and check the file to see which files you have from bos.rte.man. There should be about 11 files from that fileset.
If you don't have all the files, I'd recommend installing them from the 5.2 disks. I wouldn't copy them from a 5.1 machine to a 5.2.
And I wouldn't copy the directories from the 5.1 system's /usr/share/man. It would probably solve your current problem, but won't do you any good in the future when you might want to update your man pages.