Which hook is used to alter a field's description?
To the best of my knowledge there's no hook for that. Instead you'll want to use the following API functions in whatever context makes sense in your site:


field_info_instance()


  Returns an array of instance data for a specific field and bundle.


Use the function to load up an array of instance data/settings, which you can then alter as necessary before passing back to:
field_instance_update()


  Updates an instance of a field.


That will persist your changes to the database, and take care of clearing relevant caches.


A complete example might look like

$info = field_info_instance('node', 'body', 'article');
$info['description'] = t('New description');
field_instance_update($info);