Mention that PlanForeignModify's result must be copiable by copyObject.
baserel->fdw_private> data previously created by the
scan-planning functions. However, in INSERT> the target
table is not scanned so there is no RelOptInfo> for it.
+ The List> returned by PlanForeignModify> has
+ the same restrictions as the fdw_private> list of a
+ ForeignScan> plan node, that is it must contain only
+ structures that copyObject> knows how to copy.