Fix some documentation about FKs and partitioned tables

This got forgotten in f56f8f which has added foreign key support for
partitioned tables.  In passing, add a mention about caveats applying to
tables partitioned using inheritance regarding indexes and foreign keys.

Author: Paul A Jungwirth
Reviewed-by: Amit Langote, Michael Paquier
Discussion: https://postgr.es/m/CA+renyUuSmYgmZjKc_DfUNVZ0uttF91-FwhDVW3F7WEPj0jL5w@mail.gmail.com

doc/src/sgml/ddl.sgml

@@ -3964,14 +3964,6 @@ ALTER TABLE measurement ATTACH PARTITION measurement_y2008m02
       </para>
      </listitem>
 
-     <listitem>
-      <para>
-       While primary keys are supported on partitioned tables, foreign
-       keys referencing partitioned tables are not supported.  (Foreign key
-       references from a partitioned table to some other table are supported.)
-      </para>
-     </listitem>
-
      <listitem>
       <para>
        <literal>BEFORE ROW</literal> triggers, if necessary, must be defined
@@ -4354,6 +4346,14 @@ ALTER TABLE measurement_y2008m02 INHERIT measurement;
        </para>
       </listitem>
 
+      <listitem>
+       <para>
+        Indexes and foreign key constraints apply to single tables and not
+        to their inheritance children, hence they have some
+        <link linkend="ddl-inherit-caveats">caveats</link> to be aware of.
+       </para>
+      </listitem>
+
       <listitem>
        <para>
         The schemes shown here assume that the values of a row's key column(s)