minor #12276 Document the validator's switch to PSR-6 (derrabus)

This PR was merged into the 4.4 branch.

Discussion
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Document the validator's switch to PSR-6

This PR documents symfony/symfony#33459 and fixes #12275.

Commits
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2f010b7 Document the validator's switch to PSR-6.

Author: javiereguiluz

components/validator/resources.rst

@@ -147,17 +147,21 @@ can slow down your application because each file needs to be parsed, validated
 and converted into a :class:`Symfony\\Component\\Validator\\Mapping\\ClassMetadata`
 instance.
 
-To solve this problem, call the :method:`Symfony\\Component\\Validator\\ValidatorBuilder::setMetadataCache`
+To solve this problem, call the :method:`Symfony\\Component\\Validator\\ValidatorBuilder::setMappingCache`
 method of the Validator builder and pass your own caching class (which must
-implement :class:`Symfony\\Component\\Validator\\Mapping\\Cache\\CacheInterface`)::
+implement the PSR-6 interface :class:`Psr\\Cache\\CacheItemPoolInterface`)::
 
     use Symfony\Component\Validator\Validation;
 
     $validator = Validation::createValidatorBuilder()
         // ... add loaders
-        ->setMetadataCache(new SomeImplementCacheInterface());
+        ->setMappingCache(new SomePsr6Cache());
         ->getValidator();
 
+.. versionadded:: 4.4
+
+    Support for PSR-6 compatible mapping caches was introduced in Symfony 4.4.
+
 .. note::
 
     The loaders already use a singleton load mechanism. That means that the