Clarify that this does not verify deliverability

[fkj]

I am aware that the goal of the proposal is explicitly not to guarantee deliverability, but merely to obtain an email address. I think the text should clearly mention this (in a "Implementation Considerations" section?).

The "old-fashioned" email verification check involves sending an email to the address and waiting for a link or code to be delivered back. Since that approach does verify deliverability, I think it should be made very clear that this proposal does not. Some implementers may otherwise use the new API without considering what their needs actually are.

I am happy to provide a PR with a text suggestion if desired.