document GRASS_VECTOR_TOPO_DEBUG variable

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@64235 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/init/variables.html

@@ -282,6 +282,12 @@ PERMANENT
     map is closed or GRASS session quited. Note that this variable is
     dedicated for internal use only.</dd>
   
+  <dt>GRASS_VECTOR_TOPO_DEBUG</dt>
+  <dd>[vectorlib, v.generalize]<br> If the environment variable
+  GRASS_VECTOR_TOPO_DEBUG
+  exists, <em><a href="v.generalize.html">v.generalize</a></em> runs
+  in extremely slow debug mode.</dd>
+
   <dt>GRASS_WXBUNDLED</dt>
   <dd>[wxGUI]<br>
     set to tell wxGUI that a bundled wxPython will be used.<br>