Improve inline documentation for register_setting(). Props filosofo. fixes #10368

git-svn-id: http://svn.automattic.com/wordpress/trunk@11991 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/plugin.php

@@ -990,7 +990,8 @@ function user_can_access_admin_page() {
  *
  * @since 2.7.0
  *
- * @param string $option_group A settings group name.  Can be anything.
+ * @param string $option_group A settings group name.  Should correspond to a whitelisted option key name.
+ * 	Default whitelisted option key names include "general," "discussion," and "reading," among others. 
  * @param string $option_name The name of an option to sanitize and save.
  * @param unknown_type $sanitize_callback A callback function that sanitizes the option's value.
  * @return unknown