Add non-normative note about compromised renderer

[npm1]

Ideally, the FedCM API is used in such a way that a compromised renderer cannot access the user data before the user goes through the FedCM flow. In a world with Spectre, this may require the user agent to not have this user data in a renderer process before the user flow. This is not something that can be spec'd but we could add a note about it.