Gets the root System.Xml.XmlElement for the document.
an System.Xml.XmlElement object that represents the document element, or null if it does not exist yet.
It represents the document element (root element) of this document. A well-formed XML document must have exactly one element, but this property could be null.
This property is an equivalent of "documentElement" attribute of Document interface in W3C Document Object Model Core specification.