[DOMCore] minor wording problem for Document.importNode()

The spec current says this about the importNode() method:

> The |importNode(node, deep)| method must return a clone 
> <http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#clone> of 
> node, with new ownerDocument being the context object 
> <http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#context-object>, 
> and the /clone children flag 
> <http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#clone-children-flag>/ 
> set if deep is true. 
But the clone algorithm can throw a DATA_CLONE_ERR, so the "must return 
a clone" isn't, in general, possible.

     David