QUERY: caching - mention GET alternative wrt caching (addresses #3120)

Author: reschke

draft-ietf-httpbis-safe-method-w-body.xml

@@ -434,7 +434,8 @@ q=foo&limit=10&sort=-published
       </t>
       <t>
         The cache key for a QUERY request (see <xref target="HTTP-CACHING" section="2"/>) &MUST;
-        incorporate the request content.
+        incorporate the request content (<xref target="HTTP-CACHING" section="6"/>) and related
+        metadata (<xref target="HTTP-CACHING" section="8"/>).
       </t>
       <t>
         Caches <bcp14>MAY</bcp14> remove semantically insignificant differences first, thereby
@@ -460,6 +461,13 @@ q=foo&amp;limit=10&amp;sort=-published
         that they wish that no such transformation happens (but note that this directive is just
         advisory).
       </t>
+      <t>
+        Note that caching QUERY method responses is inherently more complex than caching responses
+        to GET, as complete reading of the request's content is needed in order to determine
+        the cache key. If a QUERY response supplies a Location response field (<xref target="location"/>)
+        to indicate a URI for an equivalent resource (<xref target="equivalent-resource"/>), clients
+        can switch to GET for subsequent requests, thereby simplifying processing.
+      </t>
     </section>
 
     <section title="Range Requests" anchor="range">