[Validation] Mention the option to pass stringable objects

validation/custom_constraint.rst

@@ -85,13 +85,19 @@ The validator class is also simple, and only has one required method ``validate(
             }
 
             if (!preg_match('/^[a-zA-Z0-9]+$/', $value, $matches)) {
+                // the argument must be a string or an object implementing __toString()
                 $this->context->buildViolation($constraint->message)
                     ->setParameter('{{ string }}', $value)
                     ->addViolation();
             }
         }
     }
 
+.. versionadded:: 4.4
+
+    The feature to allow passing an object as the ``buildViolation()`` argument
+    was introduced in Symfony 4.4.
+
 Inside ``validate``, you don't need to return a value. Instead, you add violations
 to the validator's ``context`` property and a value will be considered valid
 if it causes no violations. The ``buildViolation()`` method takes the error