r.in.xyz manual: explain import of x,y,string data trick

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@62761 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.in.xyz/r.in.xyz.html

@@ -174,6 +174,7 @@ If the raster map is to be reprojected, it may be more appropriate to reproject
 the input points with <em>m.proj</em> or <em>cs2cs</em> before running
 <em>r.in.xyz</em>.
 
+
 <h3>Interpolation into a DEM</h3>
 
 The vector engine's topographic abilities introduce a finite memory overhead
@@ -193,6 +194,13 @@ r.to.vect -z type=point in=lidar_min out=lidar_min_pt
 v.surf.rst in=lidar_min_pt elev=lidar_min.rst
 </pre></div>
 
+<h3>Import of x,y,string data</h3>
+
+<em>r.in.xyz</em> is expecting numeric values as z column. In order to 
+perform a occurence count operation even on x,y data with non-numeric 
+attribute(s), the data can be imported using either the x or y 
+coordinate as a fake z column for <b>method</b>=<tt>n</tt> (count 
+number of points per grid cell), the z values are ignored anyway.
 
 <h2>EXAMPLE</h2>