Man pages matching name curlinfo_httpauth_avail in section 3:

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