Man pages matching name gnutls_x509_dn_set_str in section 3:

gnutls_x509_dn_set_str.3.en	debian 10 / gnutls-doc
gnutls_x509_dn_set_str.3.en	debian 11 / gnutls-doc
gnutls_x509_dn_set_str.3.en	debian 12 / gnutls-doc