Docs: Add missing `@since` and `@access` tags for `WP_Date_Query::is_first_order_clause()`.

Props keesiemeijer.
#39462.

git-svn-id: https://develop.svn.wordpress.org/trunk@39672 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/date.php

@@ -251,6 +251,9 @@ class WP_Date_Query {
 	 * Checks to see if the current clause has any time-related keys.
 	 * If so, it's first-order.
 	 *
+	 * @since 4.1.0
+	 * @access protected
+	 *
 	 * @param  array $query Query clause.
 	 * @return bool True if this is a first-order clause.
 	 */