Having structured architecture conventions becomes more important when you have many architecture conversations across multiple people and would like to gain insights from the diagrams too.
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.