Add some extra comments following alert changes

Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3436)

ssl/record/ssl3_record.c

@@ -644,6 +644,10 @@ int ssl3_get_record(SSL *s)
                                 &thisrr->data[end], 1, s, s->msg_callback_arg);
         }
 
+        /*
+         * TLSv1.3 alert and handshake records are required to be non-zero in
+         * length.
+         */
         if (SSL_IS_TLS13(s)
                 && (thisrr->type == SSL3_RT_HANDSHAKE
                     || thisrr->type == SSL3_RT_ALERT)

ssl/statem/statem_lib.c

@@ -589,6 +589,10 @@ MSG_PROCESS_RETURN tls_process_key_update(SSL *s, PACKET *pkt)
         goto err;
     }
 
+    /*
+     * There are only two defined key update types. Fail if we get a value we
+     * didn't recognise.
+     */
     if (updatetype != SSL_KEY_UPDATE_NOT_REQUESTED
             && updatetype != SSL_KEY_UPDATE_REQUESTED) {
         al = SSL_AD_ILLEGAL_PARAMETER;