I know why you came here. BECAUSE IT'S HARD TO FIND CONTENT IN GITHUB WIKI!!!
Here is a solution.
My colleague Tomohisa created a nice userscript. It provides you an ability to search GitHub Wiki in pretty easy way.
Install the script from his GitHub repository. It's compatible with Firefox (with Greasemonkey, of course), Google Chrome and Safari (with Nijakit).
Then you will find search box on right above "Page History" button.
Now search all the wiki content with github-wiki-search.user.js!