https://www.mediawiki.org/wiki/Requests_for_comment/CirrusSearch
This made me realize I have a poor sense of the features in search, so I read the documentation.
http://www.mediawiki.org/wiki/Help:Searching is bare-bones, and https://en.wikipedia.org/wiki/Help:Searching disagrees. Perhaps the former describes the default MediaWiki search features, while WMF has enabled more and different features on its wikis (such as intitle: and incategory: searches).
* mw help doesn't mention the * suffix to search for partial matches, or the ~ prefix. Both work on my unmodified local wiki.
* enwiki says "Hello dolly" in quotes gives different results, mw directly contradicts this. Even on my local wiki, quotes make a difference.
* enwiki disagrees with itself what a dash in front of a word does.
I fixed mw search's explanation of the two-button search box (no longer the default) but I don't know the details on the above.
-- =S Page software engineer on Editor Engagement Experiments