Define a document's node document

Fixes #1178.

Author: jyasskin

dom.bs

@@ -4855,6 +4855,8 @@ dictionary ElementCreationOptions {
 <p>{{Document}} <a for=/>nodes</a> are simply
 known as <dfn export id=concept-document lt="document">documents</dfn>.
 
+<p>A <a>document</a>'s <a>node document</a> is itself.
+
 <p>Each <a>document</a> has an associated
 <dfn export for=Document id=concept-document-encoding>encoding</dfn> (an <a for=/>encoding</a>),
 <dfn export for=Document id=concept-document-content-type>content type</dfn> (a string),