Alter "raw scalar" pseudo array containment semantics

Previously, a raw scalar could contain a single-scalar-element ordinary
array.  Now, as before, a raw scalar may contain another raw scalar, and
an array may contain a raw scalar, but unlike before a raw scalar may
not contain an array.

This doesn't call for any new testing of the 2 GIN opclasses that make
the containment operator indexable, since JsonbContainsStrategyNumber
sets the recheck flag to true.  I have noted it separately in GIN
comments, however.