Confusing docs about GetForeignUpperPaths in fdwhandler.sgml

Hi,

I noticed that the following note about direct modification via
GetForeignUpperPaths in fdwhandler.sgml is a bit confusing. We have
another approach using PlanDirectModify, so that should be reflected in
the note as well. Please find attached a patch.

<function>PlanForeignModify</> and the other callbacks described in
<xref linkend="fdw-callbacks-update"> are designed around the
assumption
that the foreign relation will be scanned in the usual way and then
individual row updates will be driven by a local
<literal>ModifyTable</>
plan node. This approach is necessary for the general case where an
update requires reading local tables as well as foreign tables.
However, if the operation could be executed entirely by the foreign
server, the FDW could generate a path representing that and insert it
into the <literal>UPPERREL_FINAL</> upper relation, where it would
compete against the <literal>ModifyTable</> approach.

Best regards,
Etsuro Fujita