For doc block types, favor bool over the few remaining booleans

See #32444.


git-svn-id: https://develop.svn.wordpress.org/trunk@32964 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/wp-db.php

@@ -176,7 +176,7 @@ class wpdb {
 	 * @since 4.2.0
 	 * @access private
 	 * @see wpdb::check_safe_collation()
-	 * @var boolean
+	 * @var bool
 	 */
 	private $checking_collation = false;