pgsql: docs: improve pg_upgrade rsync instructions

docs: improve pg_upgrade rsync instructions

This explains how rsync accomplishes updating standby servers and
clarifies the instructions.

Reported-by: Andreas Joseph Krogh

Discussion: /messages/by-id/VisenaEmail.10.2b4049e43870bd16.15d898d696f@tc7-visena

Backpatch-through: 9.5

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2d4a614e1ec34a746aca43d6a02aa3344dcf5fd4

Modified Files
--------------
doc/src/sgml/ref/pgupgrade.sgml | 40 ++++++++++++++++++++++++----------------
1 file changed, 24 insertions(+), 16 deletions(-)

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