Mention limitation of unique in partitioned tables

Per gripe from Phil Bayer.

Authors: Amit Langote and others
Discussion: https://postgr.es/m/156236160709.1192.4498528196556144085@wrigleys.postgresql.org

doc/src/sgml/ddl.sgml

@@ -4003,6 +4003,15 @@ ALTER INDEX measurement_city_id_logdate_key
       </para>
      </listitem>
 
+     <listitem>
+      <para>
+       Unique constraints on partitioned tables must include all the
+       partition key columns.  This limitation exists because
+       <productname>PostgreSQL</productname> can only enforce
+       uniqueness in each partition individually.
+      </para>
+     </listitem>
+
      <listitem>
       <para>
        <literal>BEFORE ROW</literal> triggers, if necessary, must be defined