Ya, it really should be in the docs, its not new though, its just different than all the other endpoints.
You can find some info here: DiscourseConnect - Official Single-Sign-On for Discourse (sso)
And also how the discourse_api ruby gem uses the sync_sso endpoint: discourse_api/single_sign_on.rb at main · discourse/discourse_api · GitHub and discourse_api/sso.rb at main · discourse/discourse_api · GitHub
It will need to use the same sso secret that your sso provider is using.