v.hull manual: add note on where parameter and layer option setting

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72272 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.hull/v.hull.html

@@ -15,6 +15,13 @@ composed of triangular faces.
 Fig: Convex hull polygon created with <em>v.hull</em>
 </center>
 
+<h2>NOTES</h2>
+
+In order to use the <b>where</b> option the <b>layer</b> option must
+be set to a positive number (the default is '-1' for 'all layers'), otherwise
+all points from all layers will be used and the <b>cats</b> and <b>where</b>
+options will be ignored.
+
 <h2>EXAMPLE</h2>
 
 Example of <em>v.hull</em> 3D output (using two random 3D point