v.cluster manual: added note about cluster IDs

vector/v.cluster/v.cluster.html

@@ -85,6 +85,11 @@ Effectively, the initial seed is updated in the process. Thus separated
 networks of points are created, with each network representing a 
 cluster. The maximum distance is not used.
 
+<h2>NOTES</h2>
+
+By default, cluster IDs are stored as category values of the points
+in layer 2.
+
 <h2>EXAMPLE</h2>
 
 Analysis of random points for areas in areas of the vector