pgsql: docs: re-add instructions on setting wal_level for rsync use

docs: re-add instructions on setting wal_level for rsync use

This step was erroneously removed four days ago by me.

Reported-by: Magnus via IM

Backpatch-through: 9.5

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/00210e3fb974ff2b9affc4d8f3b29f9cb3645a60

Modified Files
--------------
doc/src/sgml/ref/pgupgrade.sgml | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

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