Where to find array keys in $display

I am looking to use this code

function hook_field_display_alter(&$display, $context) {  
  if ($context['entity_type'] == 'node' && $context['view_mode'] == 'search_index') {
    $display['label'] = 'hidden';

to amend the sequence of display fields at run time, based on field values. Doing this made me realise that I don’t know where to look to see the array keys for the $display object. Please can someone point me at the documentation that shows array keys for this and other Drupal objects. Thanks.

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/4/feed