For example, if the repo is MIT licensed, is code or docs in that project's wiki also licensed as MIT?
Does it share the same license as what is in the repo?
Do we need to be explicit about licenses in the wiki, and if so how? (e.g. a footer?)
If you consider the information in the wiki highly valuable, I think it does not hurt to indicate the license separately in the wiki as well. As you said, in calse ofthe GitHub wiki, the footer would be a good place for that license information as that is visible on all pages.