pgsql: doc PG 18 relnotes: update description of skip scan item

doc PG 18 relnotes: update description of skip scan item

Reported-by: Peter Geoghegan

Discussion: /messages/by-id/CAH2-Wz=5aGnkPsLH9u1kZED=M8TKvxX2b9XmgdOe+ePO0KxwcQ@mail.gmail.com

Backpatch-through: 18 only

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b4879afdf5bf6664a95e9591ff975ff5c0e2985a

Modified Files
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doc/src/sgml/release-18.sgml | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)