Document QFutureWatcher::isFinished behavior change

in the Qt6 changes documentation

Pick-to: 6.2
Change-Id: Ief45c145cbb9353455af39060a3fb444afe3f1b9
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/corelib/doc/src/qt6-changes.qdoc

@@ -468,6 +468,10 @@
     code that was using QFuture with QtConcurrent. Only the code that was using
     it with the undocumented \c QFutureInterface may be affected.
 
+    \li QFutureWatcher::isFinished() now reflects the finished-state of the
+    QFuture rather than returning false until QFutureWatcher::finished() has
+    been emitted.
+
     \endlist
 
     \section2 The QPromise class