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