wxGUI/psmap: note PIL requirement for preview mode

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@52242 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/docs/wxGUI.PsMap.html

@@ -12,9 +12,11 @@ user to create interactively hardcopy map outputs. This tool
 generates <em><a href="ps.map.html">ps.map</a></em> configuration file
 and then runs <em><a href="ps.map.html">ps.map</a></em> to create
 PostScript output. There are two modes - <em>Draft mode</em> for map
-composing and <em>Preview mode</em> to see how the result will look
-like. In draft mode map features (like legend or scalebar) are
-represented by a colored rectangle with a label.
+composing and <em>Preview mode</em>
+(requires <a href="http://www.pythonware.com/products/pil/">Python
+Imaging Library</a>) to see how the result will look like. In draft
+mode map features (like legend or scalebar) are represented by a
+colored rectangle with a label.
 
 <p>
 Possible output files: