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
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4131cc6b90ce274a44462627a1c878bf1950838b

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

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