Having conducting a bit of research, the closest thing to design documents are definitely what is written in your lab notebook. The research that I did involved many custom built parts, so there were many sketches and notes on design iterations in my lab notebook. But it was very different as there was no one really reviewing them. They were mostly for documenting what I did so I could go back and look at my thought process, as well as a good way to show how the design evolved over time. I never wrote a paper on what I was researching, but if I wanted to or someone wanted to in the future, then the notebook would a great place to pick up from where I left off.