The command 'which cd' prints nothing in 12.04 [duplicate]
In bash, which is an external utility. It only finds external commands: it does not know about aliases, builtins or functions. The same goes for whatis and whereis.
Forget which and use type instead.
$ type cd
cd is a shell builtin
Builtins don't have a man page of their own (unless they also exist as an external utility, but then you get the documentation of the external utility, which may support different options). They are documented in the bash manual.
See also How to use which on an aliased command? and My which command may be wrong (sometimes)?