What is the percentage of documentary films?
documentary' is a name of a category; calculation = DIVIDE(SUM(name = 'Documentary'), COUNT(film_id)) * 100
SELECT CAST(SUM(IIF(T2.name = 'Documentary', 1, 0)) AS REAL) * 100 / COUNT(T1.film_id) FROM film_category AS T1 INNER JOIN category AS T2 ON T1.category_id = T2.category_id