The union validate/1 and from_json/1 reject with belongs on the public
side: Validation is hidden, so specs referencing Validation.error() left
two unresolvable type references in the docs. Document.validation_error/0
now carries the union; Validation.error/0 aliases it. mix docs builds
with zero warnings. Operator decision (option A) ahead of the 0.1.0
publish.