Fixes #4852 - error messages involving Whits now mention Classes instead

Whits are inserted into the dependency graph in the place where an empty
class is being required. Unfortunately, when such a class is involved in
a loop, the error message shows the cycle as involving Whit[Classname].
This patch changes it to say Class[Classname], which is much easier to
understand. It also fixes puppetdoc from generating documentation on the
Whit type.