Merge branch 'auto-ml-dcgtruncation' of https://github.com/michaelgsharp/machinelearning into auto-ml-dcgtruncation

Author: michaelgsharp

src/Microsoft.ML.AutoML/Experiment/MetricsAgents/RankingMetricsAgent.cs

@@ -22,8 +22,9 @@ public RankingMetricsAgent(MLContext mlContext, RankingMetric metric, uint optim
             if (optimizationMetricTruncationLevel <= 0)
                 throw _mlContext.ExceptUserArg(nameof(optimizationMetricTruncationLevel), "DCG Truncation Level must be greater than 0");
 
-            // We want to make sure we always have at least 10 results. Getting extra results adds no measurable performance
-            // impact, so err on the side of more.
+            // We want to make sure we always report metrics for at least 10 results (e.g. NDCG@10) to the user.
+            // Producing extra results adds no measurable performance impact, so we report at least 2x of the
+            // user's requested optimization truncation level.
             _dcgTruncationLevel = optimizationMetricTruncationLevel;
         }