Mental modelling
Writing code is easy. Knowing what to write is hard. Even harder still is to understand what you need to know in the first place.
There are numerous documentation formats: request for comments (RFCs), design documents, architectural decision records (ARDs), user stories… In reality, none of them will help you to truly understand unless you get your mind deep into the problem.
And once you do that, the format almost doesn’t matter. It does matter when you are trying to communicate with others. But it doesn’t help you understand the issue better.
So instead of writing a lot of text, it is best to think harder and write less.