Let’s start with the end. When it’s over — when you make it through the marathon that is Ken Burns’s beautiful, seven-part documentary “The Roosevelts: An Intimate History,” which begins Sunday night on PBS — you may find yourself with a lingering, nebulous grief. You’re sorry it’s over. You’re sorry they’re over. You’re sorry a certain expression of American ideals is, or often appears to be, completely over. My study habits haven’t improved since college; like an idiot, I put off watching all 14 hours of “The Roosevelts” until I absolutely had to watch them on a deadline binge this...