Your question is Documenting a Feature From Scratch. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Tell me about a time you had to document a highly complex technical feature with little or no existing material to start from. How did you figure out what was true, align with the right stakeholders, and make sure the final documentation was actually useful?