minor #18391 [Serializer] Document Serializer getSupportedTypes method (ndench)

This PR was squashed before being merged into the 6.3 branch.

Discussion
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[Serializer] Document Serializer getSupportedTypes method

Closes #18042

Documented mainly from `@nicolas`-grekas comment here #18042 (comment) and tweaked the wording to match the [New in Symfony 6.3 post](https://symfony.com/blog/new-in-symfony-6-3-performance-improvements).

![2023-06-09_15-23](https://github.com/symfony/symfony-docs/assets/2062388/7cc00bcc-bb0c-49c5-9bea-680edb318ad6)

Commits
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524c05d [Serializer] Document Serializer getSupportedTypes method

Author: javiereguiluz

serializer/custom_normalizer.rst

@@ -48,6 +48,52 @@ to customize the normalized data. To do that, leverage the ``ObjectNormalizer``:
         }
     }
 
+Caching the result of supportsNormalization
+-------------------------------------------
+
+.. versionadded:: 6.3
+
+    The ``getSupportedTypes()`` method was introduced in Symfony 6.3.
+
+Both :class:`Symfony\\Component\\Serializer\\Normalizer\\NormalizerInterface` and
+:class:`Symfony\\Component\\Serializer\\Normalizer\\DenormalizerInterface` contain a new method ``getSupportedTypes()``.
+This method allows normalizers or denormalizers to declare the type of objects they can handle, and whether they are
+cacheable. With this info, even if the ``supports*()`` call is not cacheable, the Serializer can skip a ton of method calls
+to ``supports*()`` improving performance substantially in some cases.
+
+The ``getSupportedTypes()`` method should return an array where the keys represent the supported types, and the values
+indicate whether the result of the ``supports*()`` method call can be cached or not. The format of the returned array is as
+follows:
+
+- The special key ``object`` can be used to indicate that the normalizer or denormalizer supports any classes or
+  interfaces.
+- The special key ``*`` can be used to indicate that the normalizer or denormalizer might support any types.
+- The other keys in the array should correspond to specific types that the normalizer or denormalizer supports.
+- The values associated with each type should be a boolean indicating if the result of the ``supports*()`` method call for
+  that type can be cached or not. A value of ``true`` means that the result is cacheable, while ``false`` means that the
+  result is not cacheable.
+- A ``null`` value for a type means that the normalizer or denormalizer does not support that type.
+
+Here is an example of how to use the ``getSupportedTypes()`` method::
+
+    use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
+
+    class MyNormalizer implements NormalizerInterface
+    {
+        // ...
+
+        public function getSupportedTypes(?string $format): array
+        {
+            return [
+                'object' => null, // Doesn't supports any classes or interfaces
+                '*' => false, // Supports any other types, but the result is not cacheable
+                MyCustomClass::class => true, // Supports MyCustomClass and result is cacheable
+            ];
+        }
+    }
+
+Note that ``supports*()`` method implementations should not assume that ``getSupportedTypes()`` has been called before.
+
 Registering it in your Application
 ----------------------------------