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