pgsql: relnotes: clarify that PG 15 multi-xid is advanced more

relnotes: clarify that PG 15 multi-xid is advanced more

Reported-by: Peter Geoghegan

Discussion: /messages/by-id/CAH2-Wzk-oOpKObMKJ=Df4WBERX0ja14ymu3h5JrDt_FtmvH1yQ@mail.gmail.com

Backpatch-through: 15 only

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/db1fff1ea43720d7aba41e5f03a62009731e559c

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