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, the decisions that constrain it, and the conventions and operational procedures the project keeps alongside 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.