Mention QPromise on QtConcurrent page

Change-Id: I82d3ddd4a9129694d6dedcc742165cb2af135527
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/concurrent/doc/src/qtconcurrent-index.qdoc

@@ -108,6 +108,10 @@
     \li QFutureSynchronizer is a convenience class that automatically
     synchronizes several QFutures.
 
+    \li QPromise provides a way to report progress and results of the asynchronous
+    computation to QFuture. Allows suspending or canceling the task when
+    requested by QFuture.
+
     \endlist
 
     Qt Concurrent supports several STL-compatible container and iterator types,