pg_stop_backup(wait_for_archive := true) on standby server

Hi,

Since an optional second argument wait_for_archive of pg_stop_backup
has been introduced in PostgreSQL 10 we can choose whether wait for
archiving. But my colleagues found that we can do pg_stop_backup with
wait_for_archive = true on the standby server but it actually doesn't
wait for WAL archiving. Because this behavior is not documented and we
cannot find out it without reading source code it will confuse the
user.

I think we can raise an error when pg_stop_backup with
wait_for_archive = true is executed on the standby. Attached patch
change it so that.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center