MemoryDocumentOverlayCache.java: add a comment about getOverlays() for collectionGroup only being implemented for testing purposes
@@ -117,6 +117,9 @@ public class MemoryDocumentOverlayCache implements DocumentOverlayCache {
@Override
public Map<DocumentKey, Overlay> getOverlays(
String collectionGroup, int sinceBatchId, int count) {
+ // NOTE: This method is only used by the backfiller, which will not run for memory persistence;
+ // therefore, this method is being implemented only so that the test suite for
+ // `LevelDbDocumentOverlayCache` can be re-used by the test suite for this class.
SortedMap<Integer, Map<DocumentKey, Overlay>> batchIdToOverlays = new TreeMap<>();
for (Overlay overlay : overlays.values()) {
