A short and handy intro to design documentation from Colin Baird:

Design documentation is an important part of the design process. It records the thinking behind the designs. It preserves knowledge for future team members. It gives you evidence when stakeholders challenge design decisions. A screen-by-screen explanation ensures that you don’t miss anything. A decision log gives a high-level overview of the who and why of design decisions.

I'm in the process of documenting some of my design work at the moment. For example, I recently updated some content based on feedback from the clinical assurance team. So I made a note of that in our shared decision log.

Having that decision documented will help anyone new to the project understand that there is a very good reason why things are the way they are. They can be reassured that the design work was based on sound clinical guidance.

Read Baird's post if you want to get started with this kind of thing yourself. There is even a useful four-column example you can swipe for your decision log. Lovely stuff.