cadra return value

[tetomonti]

The description of the return value should be clarified. At the moment it reads as:

@return a list of key parameters that are used to cache the result of
#' permutation-based testing, a vector of permuted best scores for a given
#' \code{n_perm}, an observed best score, and a permutation p-value.

It should be more specific, listing by name each of the list components and what they represent.