pgsql: 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: /messages/by-id/20170914191250.GB6595@momjian.us

Backpatch-through: 9.5

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d2bbd6104096b1559c1683d39e2c4e4cfbcada1c

Modified Files
--------------
doc/src/sgml/ref/pgupgrade.sgml | 41 ++++++++++++++++++++++++-----------------
1 file changed, 24 insertions(+), 17 deletions(-)

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