better document handler-specific requirements

EditGeneric & AddGeneric:

- need the controller class to be:
'controller class' => 'EntityAPIController', // or subclass
- need the 'label' entity key if there are no entity properties that will get saved due to #970338: drupal_write_record() cannot save an empty (full default) row (was: saving an empty entity doesn't work).

Edit & Add:

- need 'admin ui', and an entity form and form submit handler.