Doc: clarify handling of duplicate elements in array containment tests.

Doc: clarify handling of duplicate elements in array containment tests.

The array <@ and @> operators do not worry about duplicates: if every
member of array X matches some element of array Y, then X is contained
in Y, even if several members of X get matched to the same Y member.
This was not explicitly stated in the docs though, so improve matters.

Discussion: https://postgr.es/m/156614120484.1310.310161642239149585@wrigleys.postgresql.org