Who is the audience you will be doing the documentation for?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Questions about what kind of documentation to prepare, who does it, when and so on come up fairly often. I changed the dropdown on the forum page to view postings from the last 60 days instead of 10 days and found a couple. Browse through those for anything about documents, requirements, specs.
In short, documents are created to communicate something. You have to know in pretty good detail who is going to read it, what they want to learn and what they're going to do with it. If you can answer those, the doc will write itself. Or sometimes you can just walk over and talk to the interested parties and save a lot of typing.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi