Editorial: clarify document argument of "clone a node"

This argument is only passed recursively or from importNode(). And importNode() does not work for documents, so whenever the node argument is a document, the document argument has to be the node argument.

dom.bs

@@ -4551,9 +4551,12 @@ steps:
 dom-Range-extractContents, dom-Range-cloneContents -->
 
 <ol>
- <li><p>If <var>document</var> is not given, let <var>document</var> be <var>node</var>'s
+ <li><p>If <var>document</var> is not given, then set <var>document</var> to <var>node</var>'s
  <a for=Node>node document</a>.
 
+ <li><a for=/>Assert</a>: <var>node</var> is not a <a for=/>document</a> or <var>node</var> is
+ <var>document</var>.
+
  <li>
   <p>If <var>node</var> is an <a for=/>element</a>, then:
 
@@ -4608,9 +4611,7 @@ dom-Range-extractContents, dom-Range-cloneContents -->
    <dd><p>Do nothing.
   </dl>
 
- <li><p>Set <var>copy</var>'s <a for=Node>node document</a> and <var>document</var> to
- <var>copy</var>, if <var>copy</var> is a <a>document</a>, and set <var>copy</var>'s
- <a for=Node>node document</a> to <var>document</var> otherwise.
+ <li><p>Set <var>copy</var>'s <a for=Node>node document</a> to <var>document</var>.
 
  <li><p>Run any <a>cloning steps</a> defined for <var>node</var> in
  <a>other applicable specifications</a> and pass <var>copy</var>, <var>node</var>,