Document `force_provider_authentication` not being effective when logging in with ssh public key authentication

It's currently not made clear that this option is uneffective when the user logs in with ssh with public key authentication.

We should note that at least in the [section in the authd documentation](https://documentation.ubuntu.com/authd/en/latest/howto/configure-authd/#force-remote-authentication-with-the-identity-provider), maybe also in the comment in the broker config file.

Start with the linked authd documentation section on forcing remote authentication with the identity provider, then check whether the broker config comment also describes this option. Document that force_provider_authentication is ineffective when login uses SSH public key authentication, and verify the wording is clear in each chosen location.

Written by the indexing model from the issue text.