mention SwR / SiE

Fixes #122.

draft-ietf-httpbis-cache-latest.xml

@@ -818,10 +818,11 @@
    target="cache-response-directive"/>).
 </t>
 <t>
-   A cache &MUST-NOT; send stale responses unless it is disconnected
+   A cache &MUST-NOT; generate a stale response unless it is disconnected
    (i.e., it cannot contact the origin server or otherwise find a forward
    path) or doing so is explicitly allowed (e.g., by the max-stale request
-   directive; see <xref target="cache-request-directive" />).
+   directive defined in <xref target="cache-request-directive" />, and the
+   directives defined in <xref target="RFC5861"/>).
 </t>
 </section>
 </section>