We really should mention that ? is a wildcard substitute for any character in one or more of the following pages:
https://www.mediawiki.org/wiki/Help:CirrusSearch
https://www.mediawiki.org/wiki/Help:Searching
The CirrusSearch help page should probably have a link to the Searching page, right near the top.
Also, these pages don't appear to discuss how ? works, and probably should:
https://en.wikipedia.org/wiki/Help:Searching
https://en.wikipedia.org/wiki/Help:Advanced_search
It looks like https://en.wikipedia.org/wiki/Help:Searching/Draft#Fuzzy_search.2C_wildcards.2C_and_stemming already covers ? pretty well.