Doc: Improve clarity on partitioned table limitations

Explicitly mention that primary key constraints are also included in the
limitation that the constraint columns must be a superset of the partition key
columns.

Wording suggestion from Tom Lane.

Discussion: https://postgr.es/m/64062533.78364.1601415362244@mail.yahoo.com
Backpatch-through: 11, where unique constraints on partitioned tables were added

doc/src/sgml/ddl.sgml

@@ -4056,8 +4056,8 @@ ALTER INDEX measurement_city_id_logdate_key
 
      <listitem>
       <para>
-       Unique constraints on partitioned tables must include all the
-       partition key columns.  This limitation exists because
+       Unique constraints (and hence primary keys) 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>