pgsql: relnotes: improve PG 15 schema permission change wording

relnotes: improve PG 15 schema permission change wording

Reported-by: Noah Misch

Discussion: /messages/by-id/20220630050808.GC2257984@rfd.leadboat.com

Backpatch-through: 15 only

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c69616c9386003fd62692bd33003a136e04b5e40

Modified Files
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doc/src/sgml/release-15.sgml | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)