Ahhh, thx Kovid, I didn't see it in the index and my quick glance down a few pages didn't see obvious links either. Did there not use to be some top level contents link to the API docs, or am I imagining that?
I've since found a link about 80% of the way down this page to the link you gave above:
http://manual.calibre-ebook.com/creating_plugins.html
Perhaps something more explicit could be put in the contents of that page? Rather than just a link within "The different types of plugins" section?