Fluent Interface for a XmlQueryBuilder
ToString() does too many things, it should only be calling _xDoc.ToString(); it has side-effects that make it dangerous to call more than once:

var foo = builder.ToString()
Console.WriteLine(builder.ToString());
Console.ReadLine();


Will not produce the expected output, and will not throw an exception either, resulting in an XML document with _queryNode showing up as many times as ToString() was called.