pgsql: Fix omission in partitioning limitation documentation

Fix omission in partitioning limitation documentation

UNIQUE and PRIMARY KEY constraints can be created on ONLY the
partitioned table.  We already had an example demonstrating that,
but forgot to mention it in the documentation of the limits of
partitioning.

Author: Laurenz Albe
Reviewed-By: shihao zhong, Shubham Khanna, Ashutosh Bapat
Backpatch-through: 12
Discussion: https://postgr.es/m/167299368731.659.16130012959616771853@wrigleys.postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bcaf41c608b1b7b462d6ff13c0e4fa80a8847f66

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