Specify a new document's URL

The "URL that was originally to be fetched" isn't clear about which URL
that actually is.

The choices here are:

* The request's URL: The original URL before any redirects.
* The request's current URL: The final URL after any redirects, and
  treating a Service Worker's response as from that URL.
* The response's URL: Either the final URL after any redirects, or, if a
  Service Worker intercepted the fetch and returned a response from a
  different URL, that different URL.

Fixes whatwg#3953.

source

@@ -82414,8 +82414,8 @@ interface <dfn>Location</dfn> { // but see also <a href="#the-location-interface
 
     <p><dfn data-x="set the document's address">Setting the document's address</dfn>: Any
     <code>Document</code> created by these steps must have its <span
-    data-x="concept-document-url">URL</span> set to the <span>URL</span> that was originally to be
-    fetched, ignoring any other data that was used to obtain the resource.</p>
+    data-x="concept-document-url">URL</span> set to <var>response</var>'s <span
+    data-x="concept-response-url">URL</span>.</p>
 
     <p><!--en-GB--><dfn id="initialise-the-document-object" data-x="initialize the Document object"
     data-export="">Initializing a new <code>Document</code> object</dfn>: when a