at the end of the [use-case document](https://plantuml.com/zh/use-case-diagram), we can find the usage of "rectangle", which I think is very import in use-case diagram.. But if we search for "rectangle" in that document, we can find that it is only appears at the end and there is no other place to introduce it....
So I am confused about this. Does this mean that there is more comprehensive documentation or the documentation is not perfect at all?
Thanks a lot.