allow importing existing users when the localpart matches in upstream OAuth 2.0 logins

docs/reference/configuration.md

@@ -776,6 +776,12 @@ upstream_oauth2:
         localpart:
           #action: force
           #template: "{{ user.preferred_username }}"
+          
+          # How to handle when localpart already exists.
+          # Possible values are (default: fail):
+          # - `add` : Adds the upstream account link to the existing user, regardless of whether there is an existing link or not.
+          # - `fail` : Fails the upstream OAuth 2.0 login.
+          #on_conflict: fail
 
         # The display name is the user's display name.
         displayname: