Fala ai, campeao (that's hey there, champ)!!!
You're brazilian, aren't you? If so, please join the crew of brazilian developers pursuiting this certification! If you have difficulties with english, feel free to send me your questions in portuguese (but then it would have to be in private, since the official language here in the ranch in english). Also, take the time to read the threads of discussion that go around here, they really really help.
There isn't exactly a "design decisions document", but there is a "choices" document, where you'll explain the most relevant decisions of your implementation. For instance, I chose to keep the database records in memory, and perform I/O operations only when the application starts and when it finishes, so this is something I addressed in this document. About the "user guide", you'll simply instruct people about how to use the application you built.
By the way, what's your favorite soccer team?