say how to start

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@42979 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/docs/wxGUI.Nviz.html

@@ -11,6 +11,8 @@ displaying 3D vector data in the space, and visualization
 of <em>volume</em> data (3D raster data).
 
 <p>
+To start the wxGUI Nviz extension, choose '3D view' from the map toolbar.
+<p>
 Nviz is emphasized on the ease and speed of viewer positioning and
 provided flexibility for using a wide range of data. A low resolution
 surface or wire grid (optional) provides real-time viewer positioning