Added comment regarding regexp source

Author: vanokhin

admin/class-shortcodes-ultimate-admin-settings.php

@@ -234,6 +234,7 @@ protected function get_plugin_settings() {
 	 * @since  5.0.1
 	 * @param string  $prefix Prefix value.
 	 * @return string          Sanitized string.
+	 * @see  https://developer.wordpress.org/reference/functions/add_shortcode/ Source of the RegExp.
 	 */
 	public function sanitize_prefix( $prefix ) {
 		return preg_replace( '@[<>&/\[\]\x00-\x20="\']@', '', $prefix );