QtQml: document QQmlExtensionPlugin::baseUrl

It was added here https://codereview.qt-project.org/#change,47532


without documentation.  Also updated \since because for now qdoc
seems to interpret \since 5.0 as QtQml 5.0.

Task-number: QTBUG-31928
Change-Id: I652e9471f1ad8e83b355bffc8498a6565dc9aa2b
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>