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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master

Details
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https://git.postgresql.org/pg/commitdiff/a109a0bb50ad47812b8818e9c98e4f311672d659

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