- * 'meta_key=keyname' must be also be defined. Default 'date'.
- * Accepts 'none', 'name', 'author', 'date', 'title', 'modified',
- * 'menu_order', 'parent', 'ID', 'rand', 'comment_count'.
+ * 'meta_key=keyname' must be also be defined. To sort by a
+ * specific `$meta_query` clause, use that clause's array key.
+ * Default 'date'. Accepts 'none', 'name', 'author', 'date',
+ * 'title', 'modified', 'menu_order', 'parent', 'ID', 'rand',
+ * 'RAND(x)' (where 'x' is an integer seed value),
+ * 'comment_count', 'meta_value', 'meta_value_num', 'post__in',
+ * 'post_name__in', 'post_parent__in', and the array keys
+ * of `$meta_query`.