mention signedness for convex/concave

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@41264 15284696-431f-4ddb-bdfa-cd5b030d7da7

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

@@ -29,7 +29,8 @@ Category and color table files are generated.
 Profile and tangential curvatures are the curvatures in the direction of
 steepest slope and in the direction of the contour tangent respectively. The
 curvatures are expressed as 1/metres, e.g. a curvature of 0.05 corresponds to a
-radius of curvature of 20m.
+radius of curvature of 20m. Convex form values are positive and concave form values
+are negative.
 
 <p>
 <table width="100%" border="0">