Improve docs for QFuture::then() with context

Be more precise about attaching a continuation with the default
(QtFuture::Launch::Sync) launch policy after a continuation with
context.

Change-Id: I5b80063df2443e5742033864ba012bf34ed4cdf7
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 5624b35d6514c5439b9d6dc639dc71228ca7b5ca)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp

@@ -283,3 +283,14 @@ auto future = QtConcurrent::run([] {
    // Update UI elements
 });
 //! [19]
+
+//! [20]
+QObject *context = ...;
+auto parentFuture = cachedResultsReady ? QtFuture::makeReadyFuture(results)
+                                       : QtConcurrent::run([] { /* compute results */});
+auto future = parentFuture.then(context, [] (Results results) {
+    // Runs in the context's thread
+}).then([] {
+    // May or may not run in the context's thread
+});
+//! [20]

src/corelib/thread/qfuture.qdoc

@@ -1174,6 +1174,16 @@
 
     This is because by default \c .then() is invoked from the same thread as the parent.
 
+    But note that if the continuation is attached after the parent has already finished,
+    it will be invoked in the thread where the parent future lives:
+
+    \snippet code/src_corelib_thread_qfuture.cpp 20
+
+    In the above example if \c cachedResultsReady is \c true, and a ready future is
+    returned, it is possible that the first \c .then() finishes before the second one
+    is attached. In this case it will be resolved in the current thread. Therefore, when
+    in doubt, pass the context explicitly.
+
     \note When calling this method, it should be guaranteed that the \a context stays alive
     throughout the execution of the chain.