pgsql: Doc: clarify handling of duplicate elements in array containment

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: /messages/by-id/156614120484.1310.310161642239149585@wrigleys.postgresql.org

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/216bcf583802011365b8bb038b4e73f143e857b2

Modified Files
--------------
doc/src/sgml/func.sgml | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)