Document the new early data callback and option

Document SSL_OP_NO_ANTI_REPLAY and SSL_CTX_set_allow_early_data_cb()

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6469)