Is it true that this is a plugin? Does that mean it will work with cs5 and 5.5, or is it only for Flash CS6?
Is there any documentation as far as what it does and does not support? I am assuming it does not convert all types of content in all FLA files, i.e. masking, FLVPlayback, sound, filters, etc. What does it convert to HTML5 and what can't it do right away?
Yes, CreateJS is an extension to Flash Pro CS6. The extension is supported with Flash Pro CS6 only and not with any of the erstwhile versions.
The documentation for CreateJS will be available closer to the Ship date of CS6. Please keep a tab on http://helpx.adobe.com/flash.html for the latest Help udpates.
In the meanwhile, you can check the following videos for more information:
Assets designed in Flash Pro can be exported to HTML for further development. More specifics on what can be converted to HTML5 will be available with the documentation.
Please let us know if you have any further questions.