Add docs on Markup about the goal of this class in the context of a sandbox

src/Markup.php

@@ -14,6 +14,15 @@ namespace Twig;
 /**
  * Marks a content as safe.
  *
+ * Instances of this class (and any subclass) are trusted by the Twig
+ * sandbox: method calls and property accesses on a Markup instance bypass
+ * the SecurityPolicy method/property allowlists. This is by design: Markup
+ * represents content that has already been deemed safe to output.
+ *
+ * As a consequence, when extending this class, you are responsible for
+ * ensuring that every method and property exposed by your subclass is
+ * safe to call from a sandboxed template.
+ *
  * @author Fabien Potencier <fabien@symfony.com>
  */
 class Markup implements \Countable, \JsonSerializable, \Stringable