[HttpCache] Document support for closures in Cache attribute

http_cache.rst

@@ -246,6 +246,57 @@ pattern and other request properties.
 
 Finally, for more information about expiration caching, see :doc:`/http_cache/expiration`.
 
+Computing Last-Modified and ETag with Closures
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In addition to expressions, the ``#[Cache]`` attribute allows using PHP
+closures to compute the ``Last-Modified`` and ``ETag`` headers.
+
+Closures receive two arguments:
+
+* an array containing the resolved controller arguments
+* the current ``Request`` object
+
+Example using controller arguments:
+
+.. code-block:: php-attributes
+
+    use Symfony\Component\HttpFoundation\Request;
+    use Symfony\Component\HttpKernel\Attribute\Cache;
+
+    #[Cache(
+        lastModified: static function (array $args, Request $request): \DateTimeInterface {
+            return $args['post']->getUpdatedAt();
+        },
+        etag: static function (array $args, Request $request): string {
+            return (string) $args['post']->getId();
+        }
+    )]
+    public function show(Post $post): Response
+    {
+        // ...
+    }
+
+Example using request attributes:
+
+.. code-block:: php-attributes
+
+    #[Cache(
+        lastModified: static function (array $args, Request $request): \DateTimeInterface {
+            return $request->attributes->get('date');
+        },
+        etag: static function (array $args, Request $request): string {
+            return (string) $request->attributes->get('id');
+        }
+    )]
+    public function show(): Response
+    {
+        // ...
+    }
+
+Closures follow the same rules as expressions: cache headers already set
+on the response are not overridden.
+
 .. _http-cache-validation-intro:
 
 Validation Caching