pgsql: doc PG 18 relnotes: clarify btree skip-scan item

doc PG 18 relnotes: clarify btree skip-scan item

Reported-by: Peter Geoghegan

Discussion: /messages/by-id/CAH2-Wzko57+sT=FcxHHo7jnPLhh35up_5aAvogLtj_D9bATsgQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/44ce4e1593b1821005b29ffaa19d9cbdd80747b2

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