/config/edit/maniphest.custom-field-definitions/ says

Array of custom fields for Maniphest tasks. For details on adding custom fields to Maniphest, see 'Maniphest User Guide: Adding Custom Fields'.

Going to https://secure.phabricator.com/book/phabricator/ there is no such "Maniphest User Guide" listed but "Configuration > Configuring Custom Fields" instead, pointing to https://secure.phabricator.com/book/phabricator/article/custom_fields/