Clarify handling of version conflict

This commit adds a comment explaining why version conflicts during
recovery and on replicas are considered normal and thus why we return a
successful result.

core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -487,6 +487,10 @@ private <T extends Result> Optional<T> checkVersionConflict(
                         op.versionType().explainConflictForWrites(currentVersion, expectedVersion, deleted));
                 result = onFailure.apply(e);
             } else {
+                /*
+                 * Version conflicts during recovery and on replicas are normal due to asynchronous execution; as such, we should return a
+                 * successful result.
+                 */
                 result = onSuccess.get();
             }
             return Optional.of(result);