na-adjective
documentary; documental
Describes something that is documentary in style or nature, often used for films, photography, or records. Less common than ドキュメンタリー.
この映画はドキュメンタルな手法で撮影された。
This film was shot in a documentary style.
ドキュメンタリー is the more common noun meaning 'documentary' (a film or program). ドキュメンタル is an adjectival noun describing the documentary quality or style.
Borrowed from English 'documental' or 'documentary', likely via the adjectival use.