static <T> Writer<T> doc(final Writer<? super T> writer) {
		return (xml, data) -> {
			xml.writeStartDocument("UTF-8", "1.0");
			writer.write(xml, data);
			xml.writeEndDocument();
		};
	}
Adds an XML prolog element written by the given {@code writer}. The default
values for encoding and version are set to "UTF-8" and "1.0", respectively.

<pre> {@code
<?xml version="1.0" encoding="UTF-8"?>
} </pre>

@param writer the root element writer
@param <T> the writer data type
@return a new writer instance
jenetics.xml/src/main/java/io/jenetics/xml/stream/Writer.java