Add note that PK/Unique will be the distribution key. (#2704)

gpdb-doc/dita/admin_guide/ddl/ddl-table.xml

@@ -216,6 +216,8 @@
              DISTRIBUTED RANDOMLY;
 </codeph></codeblock>
           </p>
+          <note type="important">A Primary Key is always the Distribution Key for a table. If no
+              Primary Key exists, but a Unique Key exists, this is the Distribution Key for the table.</note>
         </body>
       </topic>
     </topic>