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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a475157e6bf0c0f509cc9fb6ccff2b3876eb5f4b

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