Man pages matching name gnutls_anon_free_client_credentials in section 3:

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