In codecanyon mostly we upload templates…
… No just a game to play buyers. But if you develop your game in some framework it’s ok, but use your code, don’t copy source codes from other games. I don’t say 'you did that", just an important detail about.
Now, about document, no need to use some special document… Create a text, doc, whatever, tell some words about your game, the gameplay, how people can replace the graphics, logo, audio, etc… That’s all!
If you have license for music or graphics, just create another text, whatever and give information and link where you get it…
Good luck!