[docs] QObject: mention the return value change in 6.7

We should document it when the signature of a function changes,
incl. its return type.

Add the usual sentence.

Amends e752c77b35.

Change-Id: I694e20dec95572a128501ffc8c6e759781b4ce83
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 36effb230418427b48c9ac8d144eb102018a5291)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit dd8af5036ee41ce74e4bb8bcddf88b9e798275a6)
(cherry picked from commit d0115e99096b8dc49f7be3ca5397817deae828e6)

src/corelib/kernel/qobject.cpp

@@ -1637,6 +1637,8 @@ QThread *QObject::thread() const
     however: objects with no thread affinity can be "pulled" to the
     current thread.
 
+    In Qt versions prior to 6.7, this function had no return value (\c void).
+
     \sa thread()
  */
 bool QObject::moveToThread(QThread *targetThread QT6_IMPL_NEW_OVERLOAD_TAIL)