Improve help text at "add vocabulary" / title

Steps to reproduce: Let a drupal newbie use taxonomy... ;-)

Because of the complexity of taxonomy the help text for this field has to get improved. A whole example should be given to show up the use case for taxonomy. This example was suggested at [1];

"'Mammals' is a vocabulary whose terms could include 'dog', 'bear', 'mouse'."

[1] http://groups.drupal.org/node/7043