Document when a new session ticket gets created on resumption

When the B<gen_cb> callback is invoked, the SSL_get_session() function can be

used to retrieve the SSL_SESSION for SSL_SESSION_set1_ticket_appdata().

By default, in TLSv1.2 and below, a new session ticket is not issued on a

successful resumption and therefore B<gen_cb> will not be called. In TLSv1.3 the

default behaviour is to always issue a new ticket on resumption. In both cases

this behaviour can be changed if a ticket key callback is in use (see

L<SSL_CTX_set_tlsext_ticket_key_cb(3)>).

=head1 RETURN VALUES

The SSL_CTX_set_session_ticket_cb(), SSL_SESSION_set1_ticket_appdata() and