Writing documentation is typically an addition to writing comments. They serve different purposes. That said, adding a diagram to a comment will only make it easier to read. It is an addition. It cannot make things harder.

If only it were true, that additions cannot nake things harder to read.

In this case, it is absolutely true. It is the difference between:

  // some comment

and

  // some comment
  // additionally see ./diagram.png

Diagrams are often useful tools for communicating information.

That's not the difference I'm talking about; I'm talking about the difference between

  // some comment
  // ... 25 lines of a diagram

and

  // some comment
  // additionally see ./diagram.png