pgsql: Doc: Improve clarity on partitioned table limitations

Doc: Improve clarity on partitioned table limitations

Explicitly mention that primary key constraints are also included in the
limitation that the constraint columns must be a superset of the partition key
columns.

Wording suggestion from Tom Lane.

Discussion: https://postgr.es/m/64062533.78364.1601415362244@mail.yahoo.com
Backpatch-through: 11, where unique constraints on partitioned tables were added

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ab0c9c073563c61f9761d8d5fa02ea070a55cb85

Modified Files
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doc/src/sgml/ddl.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)