Improve warnings for small operations number (#1407)

* Improve warning about minimum iteration time

* Improve warning about multimodal distribution

* Revert "Improve warning about multimodal distribution"

This reverts commit e908289.

Author: CodeFuller

src/BenchmarkDotNet/Analysers/MinIterationTimeAnalyser.cs

@@ -25,7 +25,7 @@ protected override IEnumerable<Conclusion> AnalyseReport(BenchmarkReport report,
                 yield break;
             var minActualIterationTime = TimeInterval.FromNanoseconds(target.Min(m => m.Nanoseconds));
             if (minActualIterationTime < MinSufficientIterationTime)
-                yield return CreateWarning($"The minimum observed iteration time is {minActualIterationTime} which is very small. It's recommended to increase it.", report);
+                yield return CreateWarning($"The minimum observed iteration time is {minActualIterationTime} which is very small. It's recommended to increase it to at least {MinSufficientIterationTime} using more operations.", report);
         }
     }
 }