Which of the top results were you thinking of when you posted a link to Google? Perhaps your results are different to mine, but I had a look, and like OP I cannot find any documentation specifically to do with SSO on NGINX
in the context of DokuWiki, which is the clearly-implied question here.
As you surely know, SSO is a tricky subject at the best of times. Despite getting all sorts of other cool stuff working with a bit of googling, reading log files, backing up my server and then hacking a bit, I have been thus far unable to get SSO working on Apache - for which there is at least some sort of documentation/config/code in DokuWiki - let alone NGINX.