I recently got the task to document in detail part of a legacy software system, that does complex String building operations. The logic is mostly in one class, about 3000 lines of spagetthi-pattern code.

Main Purpose of the documentation would be to help Tier 1-2 Support resolve problems/questions that are a little bit more in depth and relieve developer resources.

I am by no means an expert documenteur, so my question is: Is there a strategy or principles i can follow to document unstructured code? Is this at all a task that can be completed in a satisfactory way? (I am thinking about recommending a refactoring, but i am quite new to the company)

I started with a flow diagramm, but with the countless if-statements it became confusing quite fast and kind of subverted its own purpose by not giving a good overview.

I have done some research, but mostly i found Infos on how to avoid unstructured code, or how to document well structured software.