Is it possible to modify help text on a vocabulary page?
When creating a vocabulary you have the option of turning on/off the ability to arrange terms in a hierarchy. If you choose "Single Hierarchy" for example then the help text will change to read: "VOCAB contains terms grouped under parent terms. You can reorganize the terms in VOCAB using their drag-and-drop handles."

See http://cgit.drupalcode.org/drupal/tree/core/modules/taxonomy/taxonomy.module#n78

... Otherwise you can create a custom module that implements hook_help and check for $route_name = entity.taxonomy_vocabulary.overview_form. As long as your module's hook_help fires after taxonomy_help() your's should win. See https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Extension%21module.api.php/function/hook_module_implements_alter/8.4.x to control whose module wins when implementing a hook