`service::users::Data::exists` is possibly bugged

https://gitlab.computer.surgery/matrix/grapevine/-/blob/5a490a4397f0c6a36dab1cb631dadc67a849deab/src/database/key_value/users.rs#L20-23

The comment says:

Check if a user has an account on this homeserver.

But it actually checks against remote users too. This is, at least, a documentation bug.