Add a TLS1.3 TODO for the msg callback

At the moment the msg callback only received the record header with the
outer record type in it. We never pass the inner record type - we probably
need to at some point.
Reviewed-by: NRich Salz <rsalz@openssl.org>

ssl/record/ssl3_record.c

@@ -209,6 +209,11 @@ int ssl3_get_record(SSL *s)
                 }
             } else {
                 /* SSLv3+ style record */
+                /*
+                 * TODO(TLS1.3): This callback only provides the "outer" record
+                 * type to the callback. Somehow we need to pass the "inner"
+                 * record type
+                 */
                 if (s->msg_callback)
                     s->msg_callback(0, 0, SSL3_RT_HEADER, p, 5, s,
                                     s->msg_callback_arg);