The design wiki is meant to hold design documentation.
Designs should be written in Markdown (supports syntax highlight) or AsciiDoc (use external syntax highlighter such as [gist] (https://gist.github.com) until github fixes the highlighting).
Try and make sure your design docs have references to JIRAs, as well as versions affected and target versions.
Pictures speak a thousand words. Use Google Drive to draw and host your images, and link to them from here.