minor #7601 Improve specification explanation (matthieu88160, javiereguiluz)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7601).

Discussion
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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.

Commits
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ce64af5 Update expiration.rst
46e6408 Update expiration.rst
b66e4d7 Update expiration.rst
13102da Update expiration.rst
8666bed Minor reword and fixed the line length
232c029 Improve specification explanation

Author: xabbuh

http_cache/expiration.rst

@@ -85,5 +85,12 @@ 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 is
+    ignored when ``s-maxage`` or ``max-age`` directives are 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#section-4.2.1