pgsql: Adjust ALTER TABLE docs on partitioned constraints

Adjust ALTER TABLE docs on partitioned constraints

Move the "additional restrictions" comment to ALTER TABLE ADD
CONSTRAINT instead of ADD CONSTRAINT USING INDEX; and in the latter
instead indicate that partitioned tables are unsupported

Noted by David G. Johnston
Discussion: https://postgr.es/m/CAKFQuwY4Ld7ecxL_KAmaxwt0FUu5VcPPN2L4dh+3BeYbrdBa5g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9a89f6d85467be362f4d426c76439cea70cd327f

Modified Files
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doc/src/sgml/ref/alter_table.sgml | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)