r.slope.aspect: aspect diagram added to the manual

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@59151 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.slope.aspect/r.slope.aspect.html

@@ -23,6 +23,10 @@ categories represent the number degrees of east and they increase
 counterclockwise: 90deg is North, 180 is West, 270 is South 360 is East. The
 aspect value 0 is used to indicate undefined aspect in flat areas with slope=0.
 
+<center>
+  <img src="aspect_diagram.png" border="0">
+</center>
+
 <p>
 The <b>slope</b> output raster map contains slope values, stated in degrees of
 inclination from the horizontal if <b>format</b>=degrees option (the default)