drupal/core/modules/migrate_drupal/migrate_drupal.module

<?php

/**
 * @file
 * Provides migration from other Drupal sites.
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function migrate_drupal_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.migrate_drupal':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Migrate Drupal module provides a framework based on the <a href="!migrate">Migrate module</a> to facilitate migration from a Drupal (6, 7, or 8) site to your website. It does not provide a user interface. For more information, see the <a href="!migrate_drupal">online documentation for the Migrate Drupal module</a>.', array('!migrate' => \Drupal::url('help.page', array('name' => 'migrate')), '!migrate_drupal' => 'https://www.drupal.org/documentation/modules/migrate_drupal')) . '</p>';
      return $output;
  }
}