Update help message for postgres targets to encourage usage of canonical script

https://tickets.ch.cam.ac.uk/rt/Ticket/Display.html?id=211460

e.g. RT 211460, 211465. spri-musuem-rt-2025 had been set up with an adhoc script
which was not properly tidying up after itself due to bailing early on a "set -e"
error.

ROOT/DEBIAN/control

@@ -3,6 +3,6 @@ Priority: optional
 Section: otherosfs
 Maintainer: Chemistry COs <support@ch.cam.ac.uk>
 Architecture: all
-Version: 0.9-ch85
+Version: 0.9-ch86
 Depends: zfs-dkms, postgresql-13 | postgresql-9.5 | postgresql-9.4 , liblockfile-simple-perl, libdbi-perl, libjson-perl, libzfs-perl-chem, libnet-openssh-perl, libdbd-pg-perl, mbuffer, rsync, nfs-kernel-server, pv, libwww-curl-perl
 Description: a backup system using ZFS (repository 'backup-scheduler')

ROOT/usr/lib/chem-zfs-backup-server/new-backup-rsnapshot

@@ -150,8 +150,15 @@ while ! [ -z $1 ] ; do
   postgres)
    backupdir $FQDN $TGT/pgdumps /var/empty 43200 60480
    echo "Now set up /etc/chem-zfs-backup-server/zfs-rsync.d/${FQDN}_.var.empty to"
-   echo "run your postgres backup task as the PRE command, and you probably also want"
-   echo "to change the prepare script in /etc/chem-zfs-backup-server/zfs-rsync.d/${FQDN}_. too."
+   echo "run your postgres backup task via e.g.:"
+   echo
+   echo "  PRE=/usr/lib/chem-zfs-backup-server/pg-dump-script"
+   echo
+   echo "You should check/etc/chem-zfs-backup-server/zfs-rsync.d/${FQDN}_. too , e.g."
+   echo
+   echo "  RSYNCARGS=\"... --exclude=/var/lib/postgresql"
+   echo "  PRE=/etc/chem-zfs-backup-server/zfs-rsync.d/prepare-ignore-mysql-and-postgres"
+   echo
    ;;
   mysql)
    backupdir $FQDN $TGT/mysql-files /var/lib/mysql-files