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
round(42.4382, 2)
42.44
+
+ round(1234.56, -1)
+ 1230