Update HltEfficiencyChecker docs for MooreAnalysis!38

Update the HltEfficiencyChecker documentation to tell people that there is a debug_mode option available.

I also take this opportunity to update some of the results and plots as the underlying rates and efficiencies have changed slightly since these docs were written. I also make a couple of small additions reflecting questions I've received in the last couple of months.

Goes with MooreAnalysis!38 (merged).