Man pages matching name gnutls_psk_free_server_credentials in section 3:

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