document -p behavior (merge from devbr6)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@40490 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.mkgrid/v.mkgrid.html

@@ -6,6 +6,9 @@ Both point and area vector grids can be created.
 
 <H2>NOTES</H2>
 
+Grid points created with the <b>-p</b> flag will be placed at the
+<i>center</i> of each grid cell, not at the grid line nodes.
+<P>
 This is NOT to be used to generate a vector map of USGS quadrangles,
 because USGS quads are not exact rectangles.