The documents at the link above are for Apache NiFi 1.1.0, but HDF 2.0.0 was built with NiFi 1.0.0. The ability to append was added to NiFi 1.1.0 under NIFI-1322, so will likely be available in an upcoming version of HDF.
The docs at that site are always for the latest version of Apache NiFi, it is recommended that you use the docs that come with your version of HDF/NiFi, using the Help option from the top-right hamburger menu in your running instance of HDF/NiFi.