Janet Jackson’s buzzy new documentary series was a hit, garnering more than 15 million viewers overall, Lifetime and A&E Network said. The four-episode series, which is titled “Janet Jackson,” averaged 3.1 million viewers between Lifetime and A&E during its premieres on the networks last Friday and Saturday, according to the announcement Tuesday. The networks say 15.7 million people tuned in for the series overall between TV viewership and digital streaming. Episodes of “Janet Jackson” covered the pop star’s childhood, career, the abuse allegations against her late brother Michael Jackson, and…