If you write it down it is documentation.

I think you're not getting the point.

It matters nothing if you write down something. For a project, only the requirements specification matters. The system requirements specification document lists exactly what you need to deliver and under which conditions. It matters nothing if you write a README.md or post something in a random wiki.

Requirements are not the same thing as specifications either, but both are documentation!

https://en.wikipedia.org/wiki/System_requirements_specification