State the documentary film titles with longest length.
documentary film refers to name = 'documentary'; longest length refers to max(length)
SELECT T1.title FROM film AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T3.category_id = T2.category_id WHERE T3.name = 'documentary' ORDER BY T1.length DESC LIMIT 1