improve SSL_CTX_set_tlsext_ticket_key_cb ref impl

improve SSL_CTX_set_tlsext_ticket_key_cb ref impl

improve reference implementation code in
  SSL_CTX_set_tlsext_ticket_key_cb man page

change EVP_aes_128_cbc() to EVP_aes_256_cbc(), with the implication
of requiring longer keys.  Updating this code brings the reference
implementation in line with implementation in openssl committed in 2016:
commit 05df5c20
Use AES256 for the default encryption algorithm for TLS session tickets

add comments where user-implementation is needed to complete code

(backport from https://github.com/openssl/openssl/pull/12063)

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12391)