Give a hint, when [] is incorrectly used for a composite type in array.

That used to be accepted, so let's try to give a hint to users on why
their PL/python functions no longer work.

Reviewed by Pavel Stehule.

Discussion: <CAH38_tmbqwaUyKs9yagyRra=SMaT45FPBxk1pmTYcM0TyXGG7Q@mail.gmail.com>