doc stdout wld file name

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@42411 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.out.png/r.out.png.html

@@ -7,7 +7,8 @@ format, respecting the current region resolution and bounds.
 Optionally the user can choose to export a World File to provide basic
 georeferencing support. When used with the transparency flag this can
 create images useful for KML, TMS, or WMS overlays. (e.g. for use in
-Google Earth or as OpenLayers tiles)
+Google Earth or as OpenLayers tiles) If output is to stdout, the world
+file will be called <tt>png_map.wld</tt>.
 
 
 <h2>SEE ALSO</h2>