Asynchronously writes the XML declaration with the version "1.0" and the standalone attribute.
standalone If true, it writes "standalone=yes"; if false, it writes "standalone=no".
The task that represents the asynchronous WriteStartDocument operation.
This is the asynchronous version of XmlWriter.WriteStartDocument(bool), with the same functionality. To use this method, you must set the XmlWriterSettings.Async flag to true.