pgsql: doc PG 18 relnotes: move and clarify constraint items

doc PG 18 relnotes: move and clarify constraint items

Reported-by: Álvaro Herrera

Discussion: /messages/by-id/202505041135.cpo7zgdcya2u@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d83981c24be76b64205158b59b6c05cf118187c3

Modified Files
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doc/src/sgml/release-18.sgml | 71 ++++++++++++++++++++++----------------------
1 file changed, 35 insertions(+), 36 deletions(-)