First things first: make sure you have a solid understanding of what you are trying to document. Ask us about it on Discord if something is unclear!
Anything under the docs folder should be proof read for any typos, lacking information or inconsistencies.
Some features may be intended to be undocumented (for instance developer only features). Typically anything that the bot lists in any command or setting listings should be documented here as well.
TL; DR
Ask us what needs doing and/or what you can do.