Clarify duplicate and invalid freshness info

Fixes #460

draft-ietf-httpbis-cache-latest.xml

@@ -723,9 +723,12 @@
 </t>
 <t>
    When there is more than one value present for a given directive (e.g., two
-   <x:ref>Expires</x:ref> header fields, multiple Cache-Control: max-age
-   directives), the directive's value is considered invalid. Caches are
-   encouraged to consider responses that have invalid freshness information to
+   <x:ref>Expires</x:ref> header field lines or multiple Cache-Control: max-age
+   directives), either the first occurrence should be used, or the response should
+   be considered stale. If directives conflict (e.g.,
+   both max-age and no-cache are present), the most restrictive directive should
+   be honored. Caches are encouraged to consider responses that have
+   invalid freshness information (e.g., a max-age directive with non-integer content) to
    be stale.
 </t>
 </section>
@@ -2491,6 +2494,7 @@
 <section title="Since draft-ietf-httpbis-cache-12" anchor="changes.since.12">
 <ul x:when-empty="None yet.">
    <li>In <xref target="storing.fields"/>, make it clear that only response headers need be stored (<eref target="https://github.com/httpwg/http-core/issues/457"/>)</li>
+   <li>In <xref target="calculating.freshness.lifetime"/> clarify how to handle invalid and conflicting directives (<eref target="https://github.com/httpwg/http-core/issues/460"/>)</li>
 </ul>
 </section>
 </section>