i.atcorr: +short explanation for input visibility raster

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72917 15284696-431f-4ddb-bdfa-cd5b030d7da7

imagery/i.atcorr/i.atcorr.html

@@ -427,6 +427,7 @@ visibility and in a following line enter the aerosol optical depth at 550nm
 </pre></div>
 
 <p>NOTE: if iaer is 0, enter -1 for visibility.
+<p>NOTE: if a visibility map is provided, these parameters are ignored.
 
 <h3>E. Target altitude (xps), sensor platform (xpp)</h3>