Documentation for ext-auth is it: /doc/acs-authentication/
It is linked from the documentation index page (/doc) as "Authentication".
You can see it on any server with acs-authentication installed. Unfortunately this does not include openacs.org, but you can go to the test servers, log in as an admin (
http://dotlrn20-test.collaboraid.net/register/admin-login), and then browse to
http://dotlrn20-test.collaboraid.net/doc/acs-authentication/ext-auth-install.html.
Did you look for it from the core docs? If so, can you remember where from, so that I can add links?
