message: Add a comment about renamed element method to docs
diff: @@ -205,6 +205,7 @@ bsoncxx::document::element element = view["name"];
  if(element.type() != bsoncxx::type::k_string) {
  // Error
  }
  +// For the versions older than 3.7.0, use get_utf8 instead of get_string
  std::string name = element.get_string().value.to_string();
  ```
  