Improve specification explanation

RFC 2616 and RFC 7234 define that a recipient MUST ignore Expires header if one of `s-max-age` or `max-age` header is defined.

Author: matthieu88160

http_cache/expiration.rst

@@ -85,5 +85,11 @@ the lifetime calculation vulnerable to clock skew. Another limitation
 of the ``Expires`` header is that the specification states that "HTTP/1.1
 servers should not send ``Expires`` dates more than one year in the future."
 
+.. note::
+
+    Accordingly with `RFC 7234 - Caching`_, the `Expires` header value will be ignored by any recipient if a `s-max-age` or `max-age` header is defined.
+
+
 .. _`expiration model`: http://tools.ietf.org/html/rfc2616#section-13.2
 .. _`FrameworkExtraBundle documentation`: https://symfony.com/doc/current/bundles/SensioFrameworkExtraBundle/annotations/cache.html
+.. _`RFC 7234 - Caching`: https://tools.ietf.org/html/rfc7234