Man pages matching name curlopt_dns_local_ip6 in section 3:

CURLOPT_DNS_LOCAL_IP6.3.en	debian 10 / libcurl4-doc
CURLOPT_DNS_LOCAL_IP6.3.en	debian 11 / libcurl4-doc
CURLOPT_DNS_LOCAL_IP6.3.en	debian 12 / libcurl4-doc