Provide a form for setting options.
Overrides TaxonomyIndexTid::buildExtraOptionsForm
public function buildExtraOptionsForm(&$form, &$form_state) {
parent::buildExtraOptionsForm($form, $form_state);
$form['depth'] = array(
'#type' => 'weight',
'#title' => t('Depth'),
'#default_value' => $this->options['depth'],
'#description' => t('The depth will match nodes tagged with terms in the hierarchy. For example, if you have the term "fruit" and a child term "apple", with a depth of 1 (or higher) then filtering for the term "fruit" will get nodes that are tagged with "apple" as well as "fruit". If negative, the reverse is true; searching for "apple" will also pick up nodes tagged with "fruit" if depth is -1 (or lower).'),
);
}