MediaWiki supports full text search. The limitations of this feature is that it indexes only words longer than 3 characters. If you want to search for shorter words (1 to 3 characters) then a known workaround is to use Google's sitesearch feature. Enter your search items in the form and it will automatically search within ArchWiki.
Of course, you can do the same manually by appending
site:wiki.archlinux.org to Google searches:
your search items site:wiki.archlinux.org
This works fine, but it is not live. Google is indexing the ArchWiki every 1 to 3 days.
You can also use DuckDuckGo's bangs feature to directly search the Arch Wiki. After adding DuckDuckGo as the default search engine of your web browser, you can directly type something like the following in the omnibox / URL bar: