An ADR (Michael Nygard, 2011) records a single architecturally significant decision as Title, Status, Context, Decision and Consequences, lightweight enough to live beside the code in source control. Once accepted it is immutable; when reality moves you write a new ADR that supersedes and links back, turning documentation into navigable decision history.
The value is the recorded rationale and trade-offs, not the diagrams. It is stage three of Decision Architecture; depth: Architecture Decision Records: The Missing Link Between Architecture and Delivery.