pgsql: Doc: last minute adjustment to the release notes

Doc: last minute adjustment to the release notes

The change made in 9d9c02ccd also affects the dense_rank() function.
Mention this in the release notes.

Author: Jonathan S. Katz
Discussion: https://postgr.es/m/5c6d3f50-e9b5-f62d-d58a-7b22eb91d8b8@postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bb76510a07f3705a28d93e065ebe4261d6ddf54d

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