Man pages matching name gnutls_x509_trust_list_add_system_trust in section 3:

gnutls_x509_trust_list_add_system_trust.3.en	debian 10 / gnutls-doc
gnutls_x509_trust_list_add_system_trust.3.en	debian 11 / gnutls-doc
gnutls_x509_trust_list_add_system_trust.3.en	debian 12 / gnutls-doc