I should write a lot more, but the two paths I see are: B.O.O. and Good Strategy/Bad Strategy.

B.O.O.: Background, Objective, Overview. Basically, a history lesson for how you got here, an objective for what you want to fix/change, and an overview of how you'll implement that change.

Good Strategy/Bad Strategy: An amazing book, but the organization is similar to boo. Problem Diagnosis, Guidelines/Assumptions/Requirements, and Actions.

I find BOO is better for targeted design documents in a google-like culture where you should write up a design document for almost any architecture change. The Good Strategy/Bad Strategy method scales pretty amazingly up to almost anything, but you need to be a much more experienced author to get things to fit it.