The idea of writing documentation before the code is interesting, it's thinking out loud
"During the testing phase, with good documentation the manager can concentrate personnel on the mistakes in the program. Without good documentation every mistake, large or small, is analyzed by one [person] who probably made the mistake in the first place because [they are] the only [person] who understands the program area."
"Following initial operations, when system improvements are in order, good documentation permits effective redesign, updating, and retrofitting in the field. If documentation does not exist, generally the entire existing framework of operating software must be junked, even for relatively modest changes."
All quotes from "Managing the Development of Large Software Systems" by Winston Royce, 1970.
All quotes perfectly match my experience working on projects that were developed using contemporary low-documentation agile development methods.
Oddly enough, I write these in Word and keep them on SharePoint. This allows the team to add comments, maintaining the historic discussion around certain functionality.