Move "Associating a Response to a Request" to Messaging.

With a mention in Architecture to explain the its role in semantics.

Fixes #145.

draft-ietf-httpbis-messaging-latest.xml

@@ -1718,6 +1718,25 @@ https://www.example.org
 </t>
 </section>
 
+<section title="Associating a Response to a Request" anchor="associating.response.to.request">
+<t>
+   HTTP/1.1 does not include a request identifier for associating a given
+   request message with its corresponding one or more response messages.
+   Hence, it relies on the order of response arrival to correspond exactly
+   to the order in which requests are made on the same connection.
+   More than one response message per request only occurs when one or more
+   informational responses (<x:ref>1xx</x:ref>, see <xref target="status.1xx"/>) precede a
+   final response to the same request.
+</t>
+<t>
+   A client that has more than one outstanding request on a connection &MUST;
+   maintain a list of outstanding requests in the order sent and &MUST;
+   associate each received response message on that connection to the highest
+   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
+   response.
+</t>
+</section>
+
 <section title="Persistence" anchor="persistent.connections">
    <x:anchor-alias value="persistent connections"/>
 <t>

draft-ietf-httpbis-semantics-latest.xml

@@ -390,6 +390,11 @@
    a message body containing the payload body (if any,
    <xref target="message.body"/>).
 </t>
+<t>
+   The mechanism used to correlate between request and response messages is
+   version dependent; some versions of HTTP use implicit ordering of messages,
+   while others use an explicit identifier.
+</t>
 <t>
    A connection might be used for multiple request/response exchanges,
    as defined in <xref target="persistent.connections"/>.
@@ -1879,25 +1884,6 @@ Host: www.example.org
 </t>
 </section>
 
-<section title="Associating a Response to a Request" anchor="associating.response.to.request">
-<t>
-   HTTP does not include a request identifier for associating a given
-   request message with its corresponding one or more response messages.
-   Hence, it relies on the order of response arrival to correspond exactly
-   to the order in which requests are made on the same connection.
-   More than one response message per request only occurs when one or more
-   informational responses (<x:ref>1xx</x:ref>, see <xref target="status.1xx"/>) precede a
-   final response to the same request.
-</t>
-<t>
-   A client that has more than one outstanding request on a connection &MUST;
-   maintain a list of outstanding requests in the order sent and &MUST;
-   associate each received response message on that connection to the highest
-   ordered request that has not yet received a final (non-<x:ref>1xx</x:ref>)
-   response.
-</t>
-</section>
-
 <section title="Message Forwarding" anchor="message.forwarding">
 <t>
    As described in <xref target="intermediaries"/>, intermediaries can serve