There's barebones documentation in the API itself (e.g. https://en.wikipedia.org/w/api.php?action=help&modules=cirrus-suggest) and scattered across Phabriator tickets, but we should write something a little more comprehensive and put it on mediawiki.org.

We may want to wait until we're more sure about what our investment in the API would be. Given our initial tests (see T109729) show great promise, I'd wager that we'll be working on this for a while, so documentation should perhaps be done sooner rather than later. I'd prioritise this lower than actually getting the first A/B test of this running, though.