Fix btagJes description
The calibration used in btagJes
isn't always the same: it's just whatever is in the input file. It's true that for pflow we were using the "T0:mc" calibration, which called whatever this file has:
("JES_MC15cRecommendation_PFlow_Aug2016_rel21.config","00-04-77","JetArea_Residual_EtaJES")
but for other jet collections it could be anything. At @svanstro's suggestion I added some footnotes that explain why we use this.