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_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e10799aa252c2232a3d4f73f4875532423f3f7b4

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