How is the api not documented? Between the docs on
Mediawiki.org and the fact that every parameter is
documented (with examples), I'd say its highly
documented.
I think he means on wiki, most people probably won't know to look
for
information on how to use it at the main/official mediawiki wiki and
just go by the scraps they can find on whatever local wiki they are on
(in this case en.wiki).