vectorintro.html: avoid potential confusion about raster MASK usage

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@57200 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/vectorintro.html

@@ -280,7 +280,9 @@ into database tables.
 
 With <a href="v.to.points.html">v.to.points</a>,
 <a href="v.to.rast.html">v.to.rast</a> and <a href="v.to.rast3.html">v.to.rast3</a>
-conversions are performed.
+conversions are performed. Note that a raster mask ("MASK") will not be
+respected since it is only applied when <em>reading</em> an existing
+GRASS raster map.
 
 <h3>Vector queries</h3>