Add paragraph about ON CONFLICT interaction with partitioning.

Author: Peter Geoghegan and Andres Freund
Discussion: CAM3SWZScpWzQ-7EJC77vwqzZ1GO8GNmURQ1QqDQ3wRn7AbW1Cg@mail.gmail.com,
    CAHGQGwFUCWwSU7dtc2aRdRk73ztyr_jY5cPOyts+K8xKJ92X4Q@mail.gmail.com
Backpatch: 9.5, where UPSERT was introduced

doc/src/sgml/ddl.sgml

@@ -3246,6 +3246,15 @@ ANALYZE measurement;
      </para>
     </listitem>
 
+    <listitem>
+     <para>
+      <command>INSERT</command> statements with <literal>ON CONFLICT</>
+      clauses are unlikely to work as expected, as the <literal>ON CONFLICT</>
+      action is only taken in case of unique violations on the specified
+      target relation, not its child relations.
+     </para>
+    </listitem>
+
    </itemizedlist>
    </para>