Man pages matching name x509_verify_param_set_flags in section 3ssl:

X509_VERIFY_PARAM_set_flags.3ssl.en	debian 10 / libssl-doc
X509_VERIFY_PARAM_set_flags.3ssl.en	debian 11 / libssl-doc
X509_VERIFY_PARAM_set_flags.3ssl.en	debian 12 / libssl-doc