Clarify valid URL

The URL standard says "A URL is a universal identifier. To disambiguate from a URL string it can also be referred to as a URL record." So it's not correct to refer to this definition of "URL" for the validity requirements of an href attribute, which contains a string, not a record.

It was intended to refer to a "URL string", which is (more or less) the valid input to the URL parser.

source

@@ -6592,8 +6592,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
 
   <h4>Terminology</h4>
 
-  <p>A <span>URL</span> is a <dfn>valid URL</dfn> if it conforms to the authoring conformance
-  requirements in the WHATWG URL standard. <ref spec=URL></p>
+  <p>A string is a <dfn>valid URL</dfn> if it is a <span>URL string</span> which conforms to
+  the authoring conformance requirements in the WHATWG URL standard. <ref spec=URL></p>
 
   <p>A string is a <dfn>valid non-empty URL</dfn> if it is a <span>valid URL</span> but it is not
   the empty string.</p>