docs:  clarify pg_upgrade docs regarding standbys and rsync
docs:  clarify pg_upgrade docs regarding standbys and rsync

Document that rsync is an _optional_ way to upgrade standbys, suggest
rsync option --dry-run, and mention a way of upgrading one standby from
another using rsync.  Also clarify some instructions by specifying if
they operate on the old or new clusters.

Reported-by: Stephen Frost, Magnus Hagander

Discussion: https://postgr.es/m/20170914191250.GB6595@momjian.us

Backpatch-through: 9.5

greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb