The limitations that it is not allowed to create/attach a foreign table
as a partition of an indexed partitioned table were not documented.

Reported-By: Stepan Yankevych
Author: Etsuro Fujita
Reviewed-By: Amit Langote
Backpatch-through: 11 where partitioned index was introduced
Discussion: https://postgr.es/m/1553869152.858391073.5f8m3n0x@frv53.fwdcdn.com