r.surf.idw: add known issue to manual

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@65215 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.surf.idw/r.surf.idw.html

@@ -78,6 +78,8 @@ the input raster map to analyze the distribution of interpolation error.
 This procedure may be helpful in choosing the number of nearest neighbors
 considered for surface generation.
 
+<h2>BUGS</h2>
+Module <em>r.surf.idw</em> works only for integer (CELL) raster maps.
 
 <h2>SEE ALSO</h2>