pgsql: pg_upgrade: improve instructions for fixing incompatible isn use

pg_upgrade: improve instructions for fixing incompatible isn use

This clarifies instructions on how to dump/reload databases that use
incompatible isn versions.

Reported-by: Alvaro Herrera

Discussion: https://postgr.es/m/20191114190652.GA23586@alvherre.pgsql

Reviewed-by: Daniel Gustafsson 

Backpatch-through: 13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f0b57aec1d41f89611efac4d6938a725b056ed00

Modified Files
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src/bin/pg_upgrade/check.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)