pgsql: doc: pg_upgrade, clarify standby servers must remain running

doc: pg_upgrade, clarify standby servers must remain running

Also mention that mismatching primary/standby LSNs should never
happen.

Reported-by: Nikolay Samokhvalov

Discussion: /messages/by-id/CAM527d8heqkjG5VrvjU3Xjsqxg41ufUyabD9QZccdAxnpbRH-Q@mail.gmail.com

Backpatch-through: 11

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/29dec9f04313fff014a43941edd1ebfeaeecb20e

Modified Files
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doc/src/sgml/ref/pgupgrade.sgml | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)