Further improve help message for new postgres backups

ROOT/DEBIAN/control

@@ -3,6 +3,6 @@ Priority: optional
 Section: otherosfs
 Maintainer: Chemistry COs <support@ch.cam.ac.uk>
 Architecture: all
-Version: 0.9-ch86
+Version: 0.9-ch87
 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

@@ -153,6 +153,7 @@ while ! [ -z $1 ] ; do
    echo "run your postgres backup task via e.g.:"
    echo
    echo "  PRE=/usr/lib/chem-zfs-backup-server/pg-dump-script"
+   echo "  PGDUMP_DATABASE=name_of_database"
    echo
    echo "You should check/etc/chem-zfs-backup-server/zfs-rsync.d/${FQDN}_. too , e.g."
    echo