standalone hot backup docs

The docs contain the following example of an archive_command for use
with standalone hot backup:

test -f /var/lib/pgsql/backup_in_progress && cp -i %p
/var/lib/pgsql/archive/%f </dev/null

Unfortunately, as I found out when I tried it, this command returns a
non-zero (on Linux/bash) when the backup_in_progress file doesn't exist,
which means that we keep accumulating WAL files when we should be
recycling them.

ISTM it would be better to use this:

if test -f /var/lib/pgsql/backup_in_progress ; then cp -i %p
/var/lib/pgsql/archive/%f </dev/null ; fi

which only returns non-zero if the copy fails.

If there's no objection I'll patch the docs accordingly - should they be
backpatched also?

cheers

andrew