"Trailers" is an HTTP/1 feature where headers can be passed on at the end of a transfer. This callback is used for when you want to send trailers with curl after an upload has been performed. An upload in the form of a chunked encoded POST.
The callback set with CURLOPT_TRAILERFUNCTION is called and the function can
then append headers to a list. One or many. When done, libcurl sends off those
as trailers to the server.