Docs: Correct and improve inline docs for parameters that accept a callback function.

See #49572

git-svn-id: https://develop.svn.wordpress.org/trunk@48473 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-setting.php

@@ -97,7 +97,7 @@ class WP_Customize_Setting {
 	 * Callback to convert a Customize PHP setting value to a value that is JSON serializable.
 	 *
 	 * @since 3.4.0
-	 * @var string
+	 * @var callable
 	 */
 	public $sanitize_js_callback = '';