pgsql: Last-minute updates for release notes.

Last-minute updates for release notes.

Security: CVE-2023-5868, CVE-2023-5869, CVE-2023-5870

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/95c96ba1d073afeadf193c7a9681a2c665dddead

Modified Files
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doc/src/sgml/release-15.sgml | 115 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 114 insertions(+), 1 deletion(-)