DOC: Replace old string formatting syntax in calling of Appender decorators
I found calling of `@Appender()` are using `%` to format string. I think it might be better to replace with `.format` based on [PEP 3101](https://www.python.org/dev/peps/pep-3101/) and [a discussion on Stack Overflow](https://stackoverflow.com/questions/5082452/string-formatting-vs-format). 

#29547 is working on replacing `%` with f-strings. This change would also help to keep the code more consistent in string template. Be more specific, the template using `%` to formate will be something like `%(XXX)s`, but using `.format` and f-strings will be the same as `{XXX}`. 