With a presidential candidate, a recent television special and 13 million adherents worldwide, the Mormons have gotten an extra dose of media attention lately. But there’s another indigenous American religion that is now making a bid for the spotlight. Formed in the 19th century, four decades after the Latter-day Saints, it, too, emphasizes a bizarre-seeming afterlife, attracts clean-cut and socially conservative adherents, encourages its members to evangelize and raises the ire of more-mainstream believers suspicious of its claims to Christianity. With “Knocking,” a documentary airing this week on PBS, director Joel Engardio draws back the curtain on America’s million Jehovah’s Witnesses.