Define 'cache key'

Fixes #728.

draft-ietf-httpbis-cache-latest.xml

@@ -264,7 +264,8 @@
 </t>
 <iref item="cache key" />
 <t>
-   The <x:dfn>cache key</x:dfn> is comprised of, at a minimum, the request method and target
+   The <x:dfn>cache key</x:dfn> is the information used to select a response and
+   is comprised of, at a minimum, the request method and target
    URI used to retrieve the stored response; the method determines under which
    circumstances that response can be used to satisfy a subsequent request. However, many
    HTTP caches in common use today only cache GET responses, and therefore only
@@ -2519,6 +2520,7 @@
 <section title="Since draft-ietf-httpbis-cache-14" anchor="changes.since.14">
 <ul x:when-empty="None yet.">
   <li>Fix subsection ordering in <xref target="cache-response-directive"/> (<eref target="https://github.com/httpwg/http-core/issues/674"/>)</li>
+  <li>In <xref target="caching.overview"/>, define what a cache key is (<eref target="https://github.com/httpwg/http-core/issues/728"/>)</li>
 </ul>
 </section>
 </section>