Clarify iteration setup/cleanup timing in xml docs.

Author: timcassell

src/BenchmarkDotNet/Engines/Engine.cs

@@ -225,7 +225,7 @@ private ClockSpan Measure(Action<long> action, long invokeCount)
             Parameters.InProcessDiagnoserHandler.Handle(BenchmarkSignal.AfterExtraIteration);
             Host.SendSignal(HostSignal.AfterExtraIteration);
 
-            data.cleanupAction(); // we run iteration cleanup after collecting GC stats
+            data.cleanupAction(); // we run iteration cleanup after diagnosers are complete.
 
             var totalOperations = data.invokeCount * Parameters.OperationsPerInvoke;
             var measurement = new Measurement(0, IterationMode.Workload, IterationStage.Extra, 1, totalOperations, clockSpan.GetNanoseconds());

src/BenchmarkDotNet/Engines/HostSignal.cs

@@ -28,12 +28,12 @@ public enum HostSignal
         AfterActualRun,
 
         /// <summary>
-        /// after actual run, before extra iteration
+        /// after actual run, after extra IterationSetup, before extra iteration
         /// </summary>
         BeforeExtraIteration,
 
         /// <summary>
-        /// after extra iteration
+        /// after extra iteration, before extra IterationCleanup
         /// </summary>
         AfterExtraIteration,