Compare Document.hidden with a boolean

The spec confused the `hidden` attribute with the `visibilityState` attribute.
The latter is a string that can take the value `"hidden"`. The former is a
boolean that is true when `visibilityState` equals `"hidden"`.

This update fixes that and drops the now useless references to the
Page Visibility spec (now integrated in HTML).

index.src.html

@@ -29,6 +29,7 @@
 
 <pre class=link-defaults>
 spec:webidl; type:interface; text:Promise
+spec:html; type:attribute; text:hidden
 </pre>
 
 <pre class='anchors'>
@@ -98,9 +99,6 @@
 
 spec: webrtc-svc; urlPrefix: https://w3c.github.io/webrtc-svc/
     type: dfn; text: scalability mode identifier; url:#scalabilitymodes*
-
-spec: visibility-state; urlPrefix: https://www.w3.org/TR/page-visibility/#
-    type: enum-value; text: hidden; url: dom-visibilitystate-hidden
 </pre>
 
 <pre class='biblio'>
@@ -5105,7 +5103,7 @@
 
 A <dfn lt="background codec|background">background codec</dfn> is a codec whose
 {{ownerDocument}} (or [=owner set=]'s {{Document}}, for codecs in workers) has a
-{{Document/hidden}} attribute equal to {{hidden|"hidden"}}.
+{{Document/hidden}} attribute equal to `true`.
 
 A User Agent MUST only [=reclaim a codec=] that is either an
 [=inactive codec=], a [=background codec=], or both. A User Agent MUST NOT