Machine Learning Design Document
By ‘design document’ I assume you are authoring a technical doc in an industry job. 

I treat this similarly to an academic journal publication but with less formatting and page restrictions. Ultimately, you want to provide sufficient information such that if Apple hired you tomorrow your replacement could reproduce the work. Further, your colleagues should be able to grasp the problem attacked, solution proposed, and evidence of success. I usually put the yawn-inducing deep stuff in appendix,  but suggested sections might include:
0) executive summary
1) background/motivation
2) data sources
3) analysis/algorithm/model
4) validation performance 
5) [optional] real world test results/pilot
6) [optional] estimated return on investment 
7) [appendix] SQL source code, modeling heuristic, outlier treatment, etc.