update comment

src/EventStore.Core/TransactionLog/Scavenging/Stages/Calculator.cs

@@ -261,9 +261,15 @@ public class Buffer {
 							break;
 
 						case DiscardDecision.AlreadyDiscarded:
-							// this event has already been deleted from the chunks but not from the index (by the old scavenger)
-							// to be on the safe side, we move the discard point forward only if all events before it
-							// have been or will be discarded
+							// this event has already been deleted from the chunks but not from the index.
+							// we move the discard point forward only if all events before it have been or will be discarded for the following reasons:
+							//
+							// i) usually, there should be no gaps between the event numbers of a stream but this property is not guaranteed
+							// with the previous scavenger. if it happens that this event was in a stream gap and we always moved the discard point
+							// forward, we would end up deleting all the events that were present before the gap.
+							//
+							// ii) we do our best to delete stale entries from the index
+
 							if (allDiscardedSoFar)
 								discardPoint = DiscardPoint.DiscardIncluding(eventInfo.EventNumber);
 							break;