Ask HN: How to visualize/document complex data processing

What are some excellent examples of visualizing or documenting complex data processes. In my specific case, I'd like to document a multi-stage ETL process with several levels of transformation and enrichment.

I know several UML diagram types that would theoretically apply for this scenario, but I feel they usually look very technical. My goal is to use the result as a documentation to get someone (and myself) quickly on speed about what's happening in the code, how data is transformed and moved around.