DOC: Clarify definition of kurtosis used
Added explation that kurtosis returned is Fisher's kurtosis, also
known as excess kurtosis.

Also cleans up the kurtosis function to remove some unused computations.

Trivial PR to better explain how kurtosis is computed.  When checking the calculation, I noticed 2 lines that were unused to also removes those.
 Indeed on the first use of `C`.
 @bashtage thanks
