Document forplay-root div id

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I was wondering how to integrate forplay into a specific div, and found out in 
the HtmlGraphics class that if a div with "forplay-root" id is set, it'll be 
used as the root forplay layer. Otherwise, the whole body will be used.

I didn't find any documentation on this subject, so maybe it should be noted 
somewhere else then in the middle of the code.

Original issue reported on code.google.com by py.ricau on 5 Jun 2011 at 7:39