Warn Studio users that XML extensions are not under the same maintenance policy as the Studio configuration

Help message on the XML extensions feature should be :

Here you can define any Nuxeo XML extension not covered by Studio editors: with little knowledge of the platform and its extension points you can go further than what Studio allows you and leverage Nuxeo products configurability even more. You'll find documentation and listing of all available extension points here (link to explorer, extension point http://explorer.nuxeo.org/nuxeo/site/distribution/current/listExtensionPoints.
Please note that Studio cannot guarantee that extensions you contribute here will be fully compatible with future versions of Nuxeo. You are responsible for validating them when upgrading your target platform.