pgsql: doc: Improve pg_upgrade rsync examples to use clusterdir

doc: Improve pg_upgrade rsync examples to use clusterdir

Commit 9521ce4a7a1125385fb4de9689f345db594c516a from Sep 13, 2017 and
backpatched through 9.5 used rsync examples with datadir.  The reporter
has pointed out, and testing has verified, that clusterdir must be used,
so update the docs accordingly.

Reported-by: Don Seiler

Discussion: https://postgr.es/m/CAHJZqBD0u9dCERpYzK6BkRv=663AmH==DFJpVC=M4Xg_rq2=CQ@mail.gmail.com

Backpatch-through: 9.5

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/06b2f15f354cadec5fb5c50bdfe9cadf24c856fb

Modified Files
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doc/src/sgml/ref/pgupgrade.sgml | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)