MINOR: add KIP-941 to Kafka Streams upgrade docs (#14577)

Reviewers: Hao Li <hli@confluent.io>, Walker Carlson <wcarlson@confluent.io>, Bill Bejeck <bill@confluent.io>

docs/streams/upgrade-guide.html

@@ -140,6 +140,13 @@
       For more information, including how it can be enabled and further configured, see the <a href="/{{version}}/documentation/streams/developer-guide/config-streams.html#rack-aware-assignment-strategy"><b>Kafka Streams Developer Guide</b></a>.
     </p>
 
+    <p>
+      IQv2 supports a <code>RangeQuery</code> that allows to specify unbounded, bounded, or half-open key-ranges. Users have to use <code>withUpperBound(K)</code>, <code>withLowerBound(K)</code>,
+      or <code>withNoBounds()</code> to specify half-open or unbounded ranges, but cannot use <code>withRange(K lower, K upper)</code> for the same.
+      <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-941%3A+Range+queries+to+accept+null+lower+and+upper+bounds">KIP-941</a> closes this gap by allowing to pass in <code>null</code>
+      as upper and lower bound (with semantics "no bound") to simplify the usage of the <code>RangeQuery</code> class.
+    </p>
+
     <h3><a id="streams_api_changes_350" href="#streams_api_changes_350">Streams API changes in 3.5.0</a></h3>
     <p>      
       A new state store type, versioned key-value stores, was introduced in