variables: document GRASS_RENDER_FILE

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@64382 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/init/variables.html

@@ -364,6 +364,9 @@ PERMANENT
   <dt>GRASS_RENDER_HEIGHT</dt>
   <dd>defines the height of output image (default is 480).</dd>
   
+  <dt>GRASS_RENDER_FILE</dt>
+  <dd>the name of the resulting image file.</dd>
+
   <dt>GRASS_RENDER_FRAME</dt>
   <dd>contains 4 coordinates, <em>top,bottom,left,right</em> (pixel
   values) with respect to the top left corner of the output image,