update manual

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@46551 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.net.alloc/v.net.alloc.html

@@ -3,8 +3,9 @@
 <em>v.net.alloc</em> allocates subnets for nearest centres (direction from 
 centre). Centre node must be opened (costs &gt;= 0). Costs of centre node are 
 used in calculation. Supported are cost assignments for both arcs and nodes, 
-and also different in both directions of a vector line. For areas cost will 
-be calculated along boundary lines.
+and also different in both directions of a vector line. For areas, costs will 
+be calculated along boundary lines. The input vector map for 
+<em>v.net.alloc</em> needs to be prepared with <em>v.net operation=connect</em>.
 
 <h2>NOTES</h2>