Man pages matching name gnutls_handshake_set_private_extensions in section 3:

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