Oauth2 missing permission/security warning #1683

Problem

The oauth2 documentation does not state which permissions are granted to an external service.
It just mention "connecting" and do not warn about possible security and data leaks.

Nextcloud allows connecting external services (for example Moodle) to your Nextcloud.

Solution

• Add a list of permissions.
• Add a security warning.

Current documentation

https://docs.nextcloud.com/server/17/admin_manual/configuration_server/oauth2.html