Man pages matching name ssl_ctx_set_client_cert_cb in section 3ssl:

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