But you don't need and official standard that everyone learns. Just use words. If your box is a class, write "class Foo". If your arrow means "contains 1 or more" just write that next to the arrow.
Yes, that’s exactly the convention that C4 specifies: add legends on every diagram and make sure things are labeled in a way that people can understand.
Well they don't follow their own rules then because the "Level 4" diagram on that page is just standard awful UML with no legend or labels.