KAFKA-3514: Upgrade Documentation (#5714)

Reviewers: Matthias J. Sax <matthias@confluent.io>

docs/streams/upgrade-guide.html

@@ -90,6 +90,7 @@
         We have also removed some public APIs that are deprecated prior to 1.0.x in 2.0.0.
         See below for a detailed list of removed APIs.
     </p>
+
     <h3><a id="streams_api_changes_210" href="#streams_api_changes_210">Streams API changes in 2.1.0</a></h3>
     <p>
         We updated <code>TopologyDescription</code> API to allow for better runtime checking.
@@ -99,6 +100,14 @@
         <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-321%3A+Update+TopologyDescription+to+better+represent+Source+and+Sink+Nodes">KIP-321</a>.
     </p>
 
+    <p>
+        We've added a new config named <code>max.task.idle.ms</code> to allow users specify how to handle out-of-order data within a task that may be processing multiple
+        topic-partitions (see <a href="/{{version}}/documentation/streams/core-concepts.html#streams_out_of_ordering">Out-of-Order Handling</a> section for more details).
+        The default value is set to <code>0</code>, to favor minimized latency over synchronization between multiple input streams from topic-partitions.
+        If users would like to wait for longer time when some of the topic-partitions do not have data available to process and hence cannot determine its corresponding stream time,
+        they can override this config to a larger value.
+    </p>
+
     <h3><a id="streams_api_changes_200" href="#streams_api_changes_200">Streams API changes in 2.0.0</a></h3>
     <p>
         We have removed the <code>skippedDueToDeserializationError-rate</code> and <code>skippedDueToDeserializationError-total</code> metrics.

streams/src/test/java/org/apache/kafka/streams/processor/internals/StreamTaskTest.java

@@ -1392,7 +1392,7 @@ public class StreamTaskTest {
             topicPartition.topic(),
             topicPartition.partition(),
             offset,
-            0L,
+            offset, // use the offset as the timestamp
             TimestampType.CREATE_TIME,
             0L,
             0,