Answer: we don't know yet, because there are Infinity ways to search.
But for action=opensearch,
http://ironholds.org/misc/api_presentation.html might make an
interesting read.
We're trying to document all the various forms of API search so that
we can do more ad-hoc and regular analysis of this stuff. There's an
awesome start at
https://www.mediawiki.org/wiki/API:Search_and_discovery that looks
pretty complete - if anyone is aware of API methods /not/ listed
there, please put them in, or reply here and I will :)
--
Oliver Keyes
Research Analyst
Wikimedia Foundation