Revise Sending a Validation Request

Fixes #110.

draft-ietf-httpbis-cache-latest.xml

@@ -875,10 +875,21 @@
 
 <section title="Sending a Validation Request" anchor="validation.sent"><iref item="validator" />
 <t>
-   When sending a conditional request for cache validation, a cache sends one
-   or more precondition header fields containing <x:dfn>validator</x:dfn>
-   metadata from its stored response(s), which is then compared by recipients
-   to determine whether a stored response is equivalent to a current
+   When generating a conditional request for validation, a cache starts with
+   either a request it is attempting to satisfy, or -- if it is initiating
+   the request independently -- it synthesises a request using a stored
+   response by copying the method, request-target, and request header fields
+   used for identifying the secondary cache key <xref
+   target="caching.negotiated.responses"/>.
+</t>
+<t>
+   It then updates that request with one or more precondition header fields.
+   These contain validator metadata sourced from stored response(s) that have
+   the same cache key (both primary and secondary, as applicable).
+</t>
+<t>
+   The precondition header fields are then compared by recipients to
+   determine whether any stored response is equivalent to a current
    representation of the resource.
 </t>
 <t>