Man pages matching name curlopt_httpproxytunnel in section 3:

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