pgsql: Doc: add example of round(v, s) with negative s.

Doc: add example of round(v, s) with negative s.

This has always worked, but you'd be unlikely to guess it
from the documentation.  Add an example showing it.

Lack of docs noted by David Johnston.  Back-patch to v13;
the documentation layout we used before that was not very
amenable to squeezing in multiple examples.

Discussion: https://postgr.es/m/CAKFQuwZ4Vy1Xty0G5Ok+ot=NDrU3C6hzF1JwUk-FEkwe3V9_RA@mail.gmail.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7ec2bfe9e0315a296a70e0f716e6dbe3ecfcd530

Modified Files
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doc/src/sgml/func.sgml | 4 ++++
1 file changed, 4 insertions(+)