message: Add note to ISAI for GPU implementation
diff: @@ -79,6 +79,11 @@ enum struct isai_type { lower, upper };
  * Incomplete Sparse Approximate Inverses for Parallel Preconditioning</a>,
  * which is the basis for this work.
  *
  + * @note GPU implementations can only handle the vector unit width `width`
  + * (warp size for CUDA) as number of elements per row in the sparse
  + * matrix. If there are more than `width` elements per row, the remaining
  + * elements will be ignored.
  + *
  * @tparam IsaiType determines if the ISAI is generated for a lower triangular
  * matrix or an upper triangular matrix
  * @tparam ValueType precision of matrix elements
  