I document as I code all the time. It is immensely helpful. As I write the signature I add a few lines saying what the function will do and often it results in me slightly modifying the function because I realize a better way.

I think you miss the point. Docs-as-code says keep the docs where the code is, and manage the docs like you manage code, using tools like git. Mostly(?) to reduce friction for software developers to actually contribute useful documentation.