pgsql: doc PG 18 relnotes: mention GROUP SET fixes

doc PG 18 relnotes: mention GROUP SET fixes

Reported-by: Richard Guo

Discussion: /messages/by-id/CAMbWs4_asKPqTCt0h9pp=zHc9vmPcnczbHeF6Xkxn1LhLapcTQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3bd5271729c5496e3712c9ffbb002a6bef99a4c3

Modified Files
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doc/src/sgml/release-18.sgml | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)