Employers usually expect to see a portfolio of work, including completed game projects or written game design documents and proposals. (creativepool.com)
At the same time the designer puts together the full game design document which describes in detail every element of the game and how it works. (creativepool.com)
Also in the book, are rare game design documents showing concepts of characters and level design that shows how the process of making a game looks. (the-gamers-lounge.com)