[Validator] Mention support of Stringable objects in Charset constraint

Author: javiereguiluz

reference/constraints/Charset.rst

@@ -5,7 +5,8 @@ Charset
 
     The ``Charset`` constraint was introduced in Symfony 7.1.
 
-Validates that a string is encoded in a given charset.
+Validates that a string (or an object implementing the ``Stringable`` PHP interface)
+is encoded in a given charset.
 
 ==========  =====================================================================
 Applies to  :ref:`property or method <validation-property-target>`