Add missing ReturnType doc to XDocumentType.WriteToAsync (#3225)

Author: krwq

xml/System.Xml.Linq/XDocumentType.xml

@@ -852,7 +852,7 @@ SystemId:Pubs.dtd
         <param name="writer">The writer to write this <see cref="T:System.Xml.Linq.XDocumentType" /> to.</param>
         <param name="cancellationToken">A token that can be used to request cancellation of the asynchronous operation.</param>
         <summary>Writes this <see cref="T:System.Xml.Linq.XDocumentType" /> to the specified <see cref="T:System.Xml.XmlWriter" />.</summary>
-        <returns>To be added.</returns>
+        <returns>A task representing the asynchronous write operation.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>