docs: WindowType is important for dialogs to center themselves

Task-number: QTBUG-41844
Task-number: QTBUG-36185
Change-Id: I61605006048e81d2666eea34f4d041124a24199f
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/gui/kernel/qwindow.cpp

@@ -1122,6 +1122,10 @@ Qt::WindowState QWindow::windowState() const
     This is a hint to the window manager that this window is a dialog or pop-up
     on behalf of the given window.
 
+    In order to cause the window to be centered above its transient parent by
+    default, depending on the window manager, it may also be necessary to call
+    setFlags() with a suitable \l Qt::WindowType (such as \c Qt::Dialog).
+
     \sa transientParent(), parent()
 */
 void QWindow::setTransientParent(QWindow *parent)