Doc: Add \readonly command to VirtualKeyboardSettings::availableLocales

Fixes: QTBUG-95664
Change-Id: Ic27fffc7e1888a9f7904716f3bb4b32eb672b4fb
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit bebc87ba

)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/virtualkeyboard/virtualkeyboardsettings.cpp

@@ -430,6 +430,7 @@ void VirtualKeyboardSettings::resetStyle()
 /*!
     \qmlproperty list<string> VirtualKeyboardSettings::availableLocales
     \since QtQuick.VirtualKeyboard.Settings 2.0
+    \readonly
 
     This property contains a list of languages supported by the virtual keyboard.