Doc: improve description of IN and row-constructor comparisons.

Doc: improve description of IN and row-constructor comparisons.

IN and NOT IN work fine on records and arrays, so just say that
they accept "expressions" not "scalar expressions".  I think that
that phrasing was meant to say that they don't work on set-returning
expressions, but that's not the common meaning of "scalar".

Revise the description of row-constructor comparisons to make it
perhaps a bit less confusing.  (This partially reverts some
dubious wording changes made by commit f56651519.)

Per gripe from Ilya Nenashev.  Back-patch to supported branches.
In HEAD and v16, also drop a NOTE about pre-8.2 behavior, which
is hopefully no longer of interest to anybody.

Discussion: https://postgr.es/m/168968062460.632.14303906825812821399@wrigleys.postgresql.org