pgsql: relnotes: update item about public schema permission change

relnotes: update item about public schema permission change

Also mention the bootstrap superuser.

Reported-by: Noah Misch

Discussion: /messages/by-id/20220628063719.GA2208244@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f5135d2aba87f59944bdab4f54129fc43a3f03d0

Modified Files
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doc/src/sgml/release-15.sgml | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)