“The people who work for the city work for you,” explains Boston Mayor Marty Walsh, to a room full of his constituents. “They’re there to service you.” That simplest of ideas, a real gimme in the not-too-distant past, stands at the center of “City Hall,” the latest of Frederick Wiseman’s documentary deep dives into the nuts and bolts of America’s institutions. He opens with a meeting between the mayor and members of the Boston Police Department, a simple strategy session: what they’re getting right, what they have to improve, what new and transformative ideas they can incorporate into their work (“What’s the follow-up on trauma?”).