Man pages matching name gnutls_x509_crt_set_key_usage in section 3:

gnutls_x509_crt_set_key_usage.3.en	debian 10 / gnutls-doc
gnutls_x509_crt_set_key_usage.3.en	debian 11 / gnutls-doc
gnutls_x509_crt_set_key_usage.3.en	debian 12 / gnutls-doc