This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I think one section on design/architecture decisions is fine. These decisions have been taken to support the functional & non-functional requirements. As reason/rationale for taking the decision you would mention the functional or non-functional requirement.