document GRASS_VERBOSE=-1 (see https://trac.osgeo.org/grass/changeset/51466)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@51467 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/init/variables.html

@@ -336,8 +336,9 @@ For specific driver-related variables see:
   <dd>[all modules]<br>
     toggles verbosity level
     <ul>
+      <li>-1 - complete silence (also errors and warnings are discarded)</li>
       <li>0 - only errors and warnings are printed</li>
-      <li>1 - progress messages are printed (percent complete)</li>
+      <li>1 - progress and important messages are printed (percent complete)</li>
       <li>2 - all module messages are printed</li>
       <li>3 - additional verbose messages are printed</li>
     </ul>