Even though the 1992 Los Angeles riots took place long before the spread of camera phones, they were captured on video from the beginning, or at least from the event that lit the spark: the beating of Rodney G. King by Los Angeles police officers. “LA 92,” one of several documentaries being released to commemorate the riots’ 25th anniversary, takes advantage of the wealth of broadcast and helicopter footage available to assemble a chronological account with a present-tense feel. The movie, which opens theatrically on Friday and airs on the National Geographic channel on Sunday, begins and ends by drawing a parallel with the 1965 Watts riots, which the news media addressed in strikingly similar language. Apart from overbearing music — the lead-in to the first day of the riots is scored with an orchestra tuning up — the film mostly lets the archival footage speak for itself.