Update documentation to reflect that refColumns isn't optional. Resolves ZF-3144

git-svn-id: http://framework.zend.com/svn/framework/standard/trunk@24820 44c647ce-9c0f-0410-b52a-842ac1e357ba

documentation/manual/en/module_specs/Zend_Db_Table-Relationships.xml

@@ -175,11 +175,12 @@ class BugsProducts extends Zend_Db_Table_Abstract
                     <command>'refColumns'</command> entry.
                 </para>
 
-                <para>
-                    It is optional to specify this element. If you don't specify the
-                    <property>refColumns</property>, the columns reported as the primary key columns
-                    of the parent table are used by default.
-                </para>
+                <note>
+                    <para>
+                        It is recommended that the <property>refColumns</property> element is always declared as 
+                        cascading operations will not work unless you do so.
+                    </para>
+                </note>
             </listitem>
 
             <listitem>