Doc: improve example for intarray's uniq() function.

Doc: improve example for intarray's uniq() function.

The previous entry invited confusion between what uniq() does
by itself and what it does when combined with sort().  The latter
usage is pretty useful so we should show it, but add an additional
example to clarify the results of uniq() alone.

Per suggestion from Martin Kalcher.  Back-patch to v13, where
we switched to formatting that supports multiple examples.

Discussion: https://postgr.es/m/165407884456.573551.8779012279828726162@wrigleys.postgresql.org