hook_help() uses routes instead of paths

The signature of hook_help() has changed to use route names instead of paths. The special route name help.page.[module_short_name] is used for the main module page. The current RouteMatch object is also passed in to hook_help() implementations, if more complex logic is needed.

Example - D7:

function dblog_help($path, $arg) {
  switch ($path) {
    case 'admin/help#dblog':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Database logging module logs system events in the Drupal database. For more information, see the online handbook entry for the <a href="@dblog">Database logging module</a>.', array('@dblog' => 'http://drupal.org/documentation/modules/dblog')) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
      $output .= '<dl>';
...
      return $output;
    case 'admin/reports/dblog':
      return '<p>' . t('The Database logging module monitors your website, capturing system events in a log (shown here) to be reviewed by an authorized individual at a later time. This log is a list of recorded events containing usage data, performance data, errors, warnings and operational information. It is vital to check the Recent log messages report on a regular basis, as it is often the only way to tell what is going on.') . '</p>';
  }

Example, D8:

use Drupal\Core\Routing\RouteMatchInterface;
...
function dblog_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.dblog':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('The Database Logging module logs system events in the Drupal database. For more information, see the online handbook entry for the <a href="!dblog">Database Logging module</a>.', array('!dblog' => 'http://drupal.org/documentation/modules/dblog')) . '</p>';
      $output .= '<h3>' . t('Uses') . '</h3>';
...
      return $output;

    case 'dblog.overview':
      return '<p>' . t('The Database Logging module monitors your website, capturing system events in a log (shown here) to be reviewed by an authorized individual at a later time. This log is a list of recorded events containing usage data, performance data, errors, warnings and operational information. It is vital to check the Recent log messages report on a regular basis, as it is often the only way to tell what is going on.') . '</p>';
  }
}

Note: add a use for RouteMatchInterface if it is not already there.