The most telling element of “20 Days in Mariupol,” Ukrainian photographer Mystylav Chernov’s pulsating documentary about the first three weeks of a Russian siege that killed tens of thousands of people, is what soldiers make of Chernov and his team. At first the AP camera crew, the last journalists left in Mariupol, are a nuisance. As apartment buildings are evacuated. Chernov is told to “turn the cameras off” by Ukrainian soldiers he politely refers to as “shy.” He replies: “This is a historical war.”