added dbref clarification

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@299699 c90b9560-bf6c-de11-be94-00142212c4b1

reference/mongo/mongodbref.xml

@@ -80,6 +80,17 @@ $people->insert(array("name" => "Fred", "likes" => array($trainRef, $soccerRef))
 ]]>
     </programlisting>
    </para>
+   <para>
+    Database references can be thought of as hyperlinks: they give the unique 
+    address of another document, but they do not load it or automatically follow
+    the link/reference.  
+   </para>
+   <para>
+    A database reference is just a normal associative array, not an instance of
+    <classname>MongoDBRef</classname>, so this class is a little different than 
+    the other data type classes.  This class contains exclusively static methods 
+    for manipulating database references.
+   </para>
   </section>
 <!-- }}} -->