Document bug regarding read only transactions

In https://www.postgresql.org/docs/10/static/hot-standby.html#hot-standby-users

It is explained that read only transactions (not in standby) allow to
update sequences.

In normal operation, <quote>read-only</> transactions are allowed to
update sequences and to use <command>LISTEN</>, <command>UNLISTEN</>, and
<command>NOTIFY</>, so Hot Standby sessions operate under slightly tighter
restrictions than ordinary read-only sessions. It is possible that some
of these restrictions might be loosened in a future release.

This is plain wrong.

BEGIN;
BEGIN
test=# SET transaction_read_only TO on;
SET
test=# SELECT nextval('t1_i_seq');
ERROR: cannot execute nextval() in a read-only transaction
test=# \q

Attached is the patch against master branch.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp