How many documentary films are rated PG-13?
documentary film refers to category.name = 'documentary'; rated PG-13 refers to rating = 'PG-13'
SELECT COUNT(T1.film_id) FROM film AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T2.category_id = T3.category_id WHERE T3.name = 'Documentary' AND T1.rating = 'PG-13'