Checks to see if all of the feed url in $check_urls are cached.
If $check_urls is empty, look for the rss feed url found in the dashboard widget options of $widget_id. If cached, call $callback, a function that echoes out output for this widget. If not cache, echo a "Loading…" stub which is later replaced by Ajax call (see top of /wp-admin/index.php)
(array) (Optional) RSS feeds
Default value: array()
(bool) False on failure. True on success.