pgsql: relnotes: update for non-exclusive backup mode removal

relnotes: update for non-exclusive backup mode removal

Reported-by: Ian Lawrence Barwick

Discussion: /messages/by-id/CAB8KJ=jmLBMAmxjcew12-yntb7m3FH-8A2+Qb7+RybZp1w9GOg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/da82c62469865e11c29881b42e955482194ca781

Modified Files
--------------
doc/src/sgml/release-15.sgml | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)