minor symfony#3959 [book][cache][tip] added cache annotations. (aitboudad)

This PR was merged into the 2.3 branch.

Discussion
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[book][cache][tip] added cache annotations.

| Q             | A
| ------------- | ---
| Doc fix?	 | no
| New docs? | yes
| Applies to | all
|Fixed tickets |	-

Commits
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a81d330 [book][cache][tip] added cache annotations.

Author: weaverryan

book/http_cache.rst

@@ -763,6 +763,12 @@ both worlds. In other words, by using both expiration and validation, you
 can instruct the cache to serve the cached content, while checking back
 at some interval (the expiration) to verify that the content is still valid.
 
+.. tip::
+
+    You can also define HTTP caching headers for expiration and validation by using
+    annotations. See the 
+    :doc:`FrameworkExtraBundle documentation </bundles/SensioFrameworkExtraBundle/annotations/cache>`.
+
 .. index::
     pair: Cache; Configuration