message: Add comments re: choice of epsilon constants.
diff: #include <vector>
  // Function to return maximum error for float and double types.
  +//
  +// Following Schatzman (1996; Accuracy of the Discrete Fourier
  +// Transform and the Fast Fourier Transform), the shape of relative
  +// l_2 error vs length should look like
  +//
  +// epsilon * sqrt(log2(length)).
  +//
  +// The magic epsilon constants below were chosen so that we get a
  +// reasonable upper bound for (all of) our tests.
  +//
  +// For rocFFT, prime lengths result in the highest error. As such,
  +// the epsilons below are perhaps too loose for pow2 lengths; but they
  +// are appropriate for prime lengths.
  template <typename Tfloat>
  inline double type_epsilon();
  template <>
  