To many Americans — millions, really — the name Fran Lebowitz doesn’t mean much. But in certain precincts, vital to the cultural functioning of both coasts, she is famously a friend, a crank, a climber, a cautionary tale, an iconoclast and a mouth. In “Public Speaking,” Martin Scorsese’s enormously enjoyable and perceptive documentary about her, Ms. Lebowitz’s endearing narcissism is a study in the notion that arrogance and insecurity are largely two sides of the same cocktail coaster.