Correct the function names in SSL_CTX_set_stateless_cookie_generate_cb.pod

Although the synopsis used the correct function names, the description did
not. Also the description of the equivalent DTLSv1_listen() callbacks was
missing, so these have been added.

Fixes #10030

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10033)