Error in DrupalCacheInterface documentation

I'm adding some revised documentation for DrupalCacheInterface. The instructions specify to set a variable called "cache_page" to override the cache_page bin.

This is incorrect because _cache_get_object() looks for a variable named "cache_class_" . $bin:

function _cache_get_object($bin) {
  // We do not use drupal_static() here because we do not want to change the
  // storage of a cache bin mid-request.
  static $cache_objects;
  if (!isset($cache_objects[$bin])) {
    $class = variable_get('cache_class_' . $bin);
    if (!isset($class)) {
      $class = variable_get('cache_default_class', 'DrupalDatabaseCache');
    }
    $cache_objects[$bin] = new $class($bin);
  }
  return $cache_objects[$bin];
}

The revised documentation mentions the correct naming convention.

I also added a block about creating a fully custom cache bin. I think that this will be as common as overriding Drupal's cache bins. The extra documentation will help.