What documentation is included in the box with the iPad 3?
I just checked the box for my iPad 2. There is essentially nothing other than:


a tiny card with minimal instructions. This tells you that there is further documentation at help.apple.com/ipad, along with information about needing to download iTunes, etc.
The obligatory "Important Product Information Guide" containing a whole lot of legal stuff. 
Some Apple stickers. 


So I think your best bet for information would be the iPad user manual linked in the other answer.

A couple of photos of the card are shown below:



Please note that this is from the iPad 2 so this may have changed since I bought mine. Although Apple has been moving towards less and less documentation over the years.