Man pages matching name ssl_ctx_add_extra_chain_cert in section 3ssl:

SSL_CTX_add_extra_chain_cert.3ssl.en	debian 10 / libssl-doc
SSL_CTX_add_extra_chain_cert.3ssl.en	debian 11 / libssl-doc
SSL_CTX_add_extra_chain_cert.3ssl.en	debian 12 / libssl-doc