message: add little doc to vals_c
diff: @@ -119,6 +119,10 @@ class json_data : public stan::io::var_context {
  return empty_vec_r_;
  }
  + /**
  + * Read out a vector of complex numbers from the specifed name.
  + * @param name Name of Variable.
  + */
  std::vector<std::complex<double>> vals_c(const std::string &name) const {
  if (contains_r_only(name)) {
  auto&& vec_r = (vars_r_.find(name)->second);
  