Man pages matching name gnutls_dtls_set_mtu in section 3:

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