minor #21654 [HttpKernel] Split cache examples (as it's not repeatable) (tcoch)

This PR was merged into the 6.4 branch.

Discussion
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[HttpKernel] Split cache examples (as it's not repeatable)

Since `#[Cache]` attribute is not repeatable, I propose to correct the example in the doc with this new version in order to avoid in confusion

Commits
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b9cec7864 Split cache examples

http_cache/cache_vary.rst

@@ -41,8 +41,13 @@ attribute::
         // ...
 
         #[Cache(vary: ['Accept-Encoding'])]
+        public function foo(): Response
+        {
+            // ...
+        }
+
         #[Cache(vary: ['Accept-Encoding', 'User-Agent'])]
-        public function index(): Response
+        public function bar(): Response
         {
             // ...
         }