Comedies often defy the concept of the sophomore slump and actually improve in their second seasons as the writers learn how to write for the timing of their characters, and everyone gains confidence through the mandate of a renewal. It’s hard to believe that FX’s “Reservation Dogs,” one of the best comedies of 2021, could get better, but the first four episodes of season two display little sign of decline, hinting at a year that could be even emotionally richer than the first.