Man pages matching name curlopt_upload_buffersize in section 3:

CURLOPT_UPLOAD_BUFFERSIZE.3.en	debian 10 / libcurl4-doc
CURLOPT_UPLOAD_BUFFERSIZE.3.en	debian 11 / libcurl4-doc
CURLOPT_UPLOAD_BUFFERSIZE.3.en	debian 12 / libcurl4-doc