doc: clarify which table creation is used for inheritance part.

Previously people might assume that the partition syntax version of
CREATE TABLE is to be used for the inheritance partition table example;
mention that the non-partitioned version should be used.

Reported-by: mib@nic.at

Discussion: https://postgr.es/m/158089540905.1098.15071165437284409576@wrigleys.postgresql.org

Backpatch-through: 10

doc/src/sgml/ddl.sgml

@@ -4103,8 +4103,8 @@ ALTER INDEX measurement_city_id_logdate_key
      <title>Example</title>
 
      <para>
-      We use the same <structname>measurement</structname> table we used
-      above.  To implement partitioning using inheritance, use
+      We use the non-partitioned <structname>measurement</structname>
+      table above.  To implement partitioning using inheritance, use
       the following steps:
 
       <orderedlist spacing="compact">