explain that display frame coordinates are measured from top-left corner

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@63408 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/init/variables.html

@@ -344,7 +344,7 @@ PERMANENT
   <dd>defines the height of output image (default is 480).</dd>
   
   <dt>GRASS_RENDER_FRAME</dt>
-  <dd>contains 4 coordinates, <em>top,bottom,left,right</em> (pixel values), defining the initial frame.</dd>
+  <dd>contains 4 coordinates, <em>top,bottom,left,right</em> (pixel values) from top-left corner, defining the initial frame.</dd>
 
   <dt>GRASS_RENDER_LINE_WIDTH</dt>
   <dd>defines default line width.</dd>