Doc: explain how to always show ScrollBar when needed

Until we e.g. add a transient property to ScrollBar, we should document
how users can ensure that it is always visible when the content size
exceeds the view size.

Task-number: QTBUG-76895
Change-Id: Iaf755f51ca9fd3865ae710a2cba13b58b897930e
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit 62e3830944)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/quickcontrols/doc/snippets/qtquickcontrols-scrollbar-policy-alwayson-when-needed.qml

@@ -0,0 +1,21 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
+
+import QtQuick
+import QtQuick.Controls
+
+ListView {
+    id: listView
+    anchors.fill: parent
+    model: 20
+    delegate: ItemDelegate {
+        width: listView.width
+        text: modelData
+    }
+
+    ScrollBar.vertical: ScrollBar {
+        //! [1]
+        policy: listView.contentHeight > listView.height ? ScrollBar.AlwaysOn : ScrollBar.AlwaysOff
+        //! [1]
+    }
+}

src/quicktemplates/qquickscrollbar.cpp

@@ -656,7 +656,17 @@ void QQuickScrollBar::resetInteractive()
 
     The following example keeps the vertical scroll bar always visible:
 
-    \snippet qtquickcontrols-scrollbar-policy.qml 1
+    \snippet qtquickcontrols-scrollbar-policy-alwayson.qml 1
+
+    Styles may use this property in combination with the \l active property
+    in order to implement transient scroll bars. Transient scroll bars are
+    hidden shortly after the last interaction event (hover or press). This
+    is typically done by animating the opacity of the scroll bar. To override
+    this behavior, set the policy to \c ScrollBar.AlwaysOn or
+    \c ScrollBar.AlwaysOff, depending on the size of the content compared to
+    its view. For example, for a vertical \l ListView:
+
+    \snippet qtquickcontrols-scrollbar-policy-alwayson-when-needed.qml 1
 */
 QQuickScrollBar::Policy QQuickScrollBar::policy() const
 {