pgsql: docs: Fix pg_upgrade standby server upgrade docs

docs: Fix pg_upgrade standby server upgrade docs

It was unsafe to instruct users to start/stop the server after
pg_upgrade was run but before the standby servers were rsync'ed. The
new instructions avoid this.

RELEASE NOTES: This fix should be mentioned in the minor release notes.

Reported-by: Dmitriy Sarafannikov and Sergey Burladyan

Discussion: https://postgr.es/m/87wp8o506b.fsf@seb.koffice.internal

Backpatch-through: 9.5, where standby server upgrade instructions first appeared

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fd376afc9863dd8ea3eba95edfa79961173e706f

Modified Files
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doc/src/sgml/ref/pgupgrade.sgml | 33 +++++++++++++++------------------
1 file changed, 15 insertions(+), 18 deletions(-)