Living Project Documentation

Use this capability to keep a project's own documentation — what the product is, the language its domain speaks, how it currently behaves, and the conventions and operational procedures the project keeps alongside them, each carrying the constraints that bind it and the reasoning behind them — true as the code changes, and to read it before planning a change rather than discovering it afterwards.

The weight is on upkeep. Documentation that is only ever created is a liability: it accumulates claims nobody re-checks, and a reader cannot tell which of them still hold. What makes it worth keeping is a mechanism that corrects it, tied to the change that invalidated it. That mechanism is the whole subject here; creating documents is the small part.