foreach (XElement item in doc.Descendants("NodeName"))
{
XElement ObjElement= item .Element("ElementName");
XComment comm = new XComment("Add Comment Here");
xe1.AddBeforeSelf(comm);
}
Iterate through the root nodes and go to your element. By using XElement'S object.AddBeforeSelf() method we can add our comments. and save the XDocument
Thank you all