update manual, add message

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@50641 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.rectify/main.c

@@ -244,6 +244,7 @@ int main(int argc, char *argv[])
     if (!no_topo->answer)
 	Vect_build(&Out);
     /* Copy tables */
+    G_message(_("Copying attribute table(s)..."));
     if (Vect_copy_tables(&In, &Out, 0))
         G_warning(_("Failed to copy attribute table to output map"));
     Vect_close(&Out);

vector/v.rectify/v.rectify.html

@@ -8,6 +8,14 @@ map with a transformed coordinate system (i.e., a different coordinate
 system than before it was rectified).
 
 <p>
+Great care should be taken with the placement of Ground Control Points. 
+For 2D transformation, the control points must not lie on a line, instead 
+3 of the control points must form a triangle. For 3D transformation, the 
+control points must not lie on a plane, instead 4 of the control points 
+must form a triangular pyramid. It is recommended to investigate RMS 
+errors and deviations of the Ground Control Points prior to transformation.
+
+<p>
 2D Ground Control Points can be identified in 
 <em><a href="i.points.html">i.points</a></em>
 or