I’ve been using MonoDraw https://monodraw.helftone.com/ for this exact sort of stuff for years, was one of the main reasons I bought the program actually. A good diagram in a block comment right above or below done complicated code is a ready powerful way to make sure you’re going to be able to maintain that code in the future.

I find flowchart style diagrams for finite state machines and UML(ish) sequence diagrams for RPC code are both extremely handy places to add a diagram.