A woman sits with her grandson by a window, peacefully sewing as thunder roars outside. She explains to him that it reminds her of a stormy fishing trip his grandpa once made during wartime. As naval ships fired rockets, she says it looked like there was fire at sea. For most filmmakers, this anecdotal story would seem to be unimportant, probably to be discarded on the cutting room floor. But for Gianfranco Rosi, it gave him the title for his latest award-winning documentary “Fire at Sea,” an uncommonly observant film.