I am a bit lost. If you don't enter any sync information, nothing will leave your machine. (People with personal (internal -> external) firewalls like LittleSnitch (on macOS, don't know about Win) can see that nothing is happening.) There's one option in settings Save geo-location with notes that triggers a request to a public API to retrieve the location.
Oh, and check for updates will connect to github to check for new releases. But you can turn that off too.
When it comes to the add-on, Laurent explained at one point what the permissions are for. But it should be obvious. If you want the clipper to clip web pages, it will need access to those web pages. That's the only permission needed.
What is the 3rd party telemetry you are talking about? There's no tracking in the app, nor does the app transfer any data out of the app to other servers, unless you sync to an external server, but in that case the transfer is wanted.
So what exactly do youi think is missing in the documentation? You can always open a PR with the info you think is missing.