doc: Mention that Window.visible is false by default

Fixes: QTBUG-120349
Pick-to: 6.6 6.5 6.2 5.15
Change-Id: I070927d0bd8b454925b4a2ab0ddf259d47d31335
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 6efc9f1406)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/quick/items/qquickwindow.cpp

@@ -3522,6 +3522,8 @@ void QQuickWindow::endExternalCommands()
 
     Setting visible to false is the same as setting \l visibility to \l {QWindow::}{Hidden}.
 
+    The default value is \c false, unless overridden by setting \l visibility.
+
     \sa visibility
  */