Doc: Update FDW documentation about GetForeignUpperPaths().

In commit d50d172e, which added support for LIMIT/OFFSET pushdown in
postgres_fdw, a new struct was introduced as the extra parameter of
GetForeignUpperPaths() set for UPPERREL_FINAL, but I forgot to update
the documentation to mention that.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/CAPmGK17uSXQDe31oRb-z1nYyT6vVzkstZkA3_Wbq38U92b9BmQ%40mail.gmail.com

doc/src/sgml/fdwhandler.sgml

@@ -387,7 +387,9 @@ GetForeignUpperPaths(PlannerInfo *root,
      step.  The <literal>extra</literal> parameter provides additional details,
      currently, it is set only for <literal>UPPERREL_PARTIAL_GROUP_AGG</literal>
      or <literal>UPPERREL_GROUP_AGG</literal>, in which case it points to a
-     <literal>GroupPathExtraData</literal> structure.
+     <literal>GroupPathExtraData</literal> structure;
+     or for <literal>UPPERREL_FINAL</literal>, in which case it points to a
+     <literal>FinalPathExtraData</literal> structure.
      (Note that <structname>ForeignPath</structname> paths added
      to <literal>output_rel</literal> would typically not have any direct dependency
      on paths of the <literal>input_rel</literal>, since their processing is expected