doc: mention inheritance's tableoid can be used in partitioning

Previously tableoid was not mentioned in the partition doc section.  We
only had a link to the "all the normal rules" of inheritance section.

Reported-by: michal.palenik@freemap.sk

Discussion: https://postgr.es/m/162627031219.693.11508199541771263335@wrigleys.postgresql.org

Backpatch-through: 10

Author: bmomjian

doc/src/sgml/ddl.sgml

@@ -4105,7 +4105,8 @@ ALTER INDEX measurement_city_id_logdate_key
 
     <para>
      Since a partition hierarchy consisting of the partitioned table and its
-     partitions is still an inheritance hierarchy, all the normal rules of
+     partitions is still an inheritance hierarchy,
+     <structfield>tableoid</structfield> and all the normal rules of
      inheritance apply as described in <xref linkend="ddl-inherit"/>, with
      a few exceptions: