Copy document's allow declarative shadow roots

Fixes #1366.

Author: annevk

dom.bs

@@ -4920,9 +4920,8 @@ and an optional <a for=/>document</a> <dfn export for="clone a node"><var>docume
    <dt>{{Document}}
    <dd><p>Set <var>copy</var>'s <a for=Document>encoding</a>, <a for=Document>content type</a>,
    <a for=Document>URL</a>, <a for=Document>origin</a>, <a for=Document>type</a>,
-   <a for=Document>mode</a>, and <a for=Document>custom element registry</a> to those of
-   <var>node</var>.
-   <!-- XXX https://github.com/whatwg/dom/issues/1366 -->
+   <a for=Document>mode</a>, <a for=Document>allow declarative shadow roots</a>, and
+   <a for=Document>custom element registry</a> to those of <var>node</var>.
 
    <dt>{{DocumentType}}
    <dd><p>Set <var>copy</var>'s <a for=DocumentType>name</a>, <a>public ID</a>, and