say that 3D input is needed

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@48490 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.outlier/v.outlier.html

@@ -1,6 +1,6 @@
 <h2>DESCRIPTION</h2>
 
-<em>v.outlier</em> removes outlier points. By default, the outlier
+<em>v.outlier</em> removes outliers in a 3D point cloud. By default, the outlier
 identification is done by a bicubic spline interpolation of the
 observation with a high regularization parameter and a low resolution
 in south-north and east-west directions. Those points that differ in