feature #19352 [Validator] Mention support of Stringable objects in Charset constraint (javiereguiluz)

This PR was squashed before being merged into the 7.1 branch.

Discussion
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[Validator] Mention support of Stringable objects in Charset constraint

Fixes #19350.

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6a36a22 [Validator] Mention support of Stringable objects in Charset constraint

Author: OskarStark

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>`