small doc change.

docs/graph.html

@@ -1676,6 +1676,7 @@ var options: {
     The graph can be used to display nodes in a hierarchical way. This can be determined automatically, based on the amount of edges connected to each node, or defined by the user.
     If the user wants to manually determine the hierarchy, each node has to be supplied with a level (from 0 being heighest to n). The automatic method
     is shown in <a href="../examples/graph/23_hierarchical_layout.html">example 23</a> and the user-defined method is shown in <a href="../examples/graph/24_hierarchical_layout_userdefined.html">example 24</a>.
+    This layout method does not support smooth curves or clustering. It automatically turns these features off.
 </p>
 
 <pre class="prettyprint">