Update _webform_theme_component() to show that path parameter is required

Since everything other callback is hooked up to the module that defines the component, it would make sense to document that you need to supply the path parameter in the API for this function to make it obvious that you are simply providing a wrapper for third party modules and that you are not automating this too. (Or better define path for the module if file defined when parsing these details - an API change maybe now best left to 8.x branch)

Maybe:

/**
 * Module specific instance of hook_theme().
 *
 * This allows each Webform component to add information into hook_theme().
 * If you specify a file to include, you must define the path to the module that this
 * file belongs to.
 */
function _webform_theme_component() {
  return array(
    'webform_grid' => array(
      'render element' => 'element',
      'file' => 'components/grid.inc',
      'path' => drupal_get_path('module', 'webform'),
    ),
    'webform_display_grid' => array(
      'render element' => 'element',
      'file' => 'components/grid.inc',
      'path' => drupal_get_path('module', 'webform'),
    ),
  );
}